All the wheel bearings I have seen fail over the years have all been the inside ones on the hub .
Myself ,I have drilled and taped the hub with a grease nipple and fill from there so grease gets into both bearings and have never had a problem.
Also have put a strip of galvanized metal, with a hole at each end for wheel studs put over cap then put on the wheel nuts to lock the cap on .Put a small hole in the cap so grease can escape when you fill up from nipple.
Done this because sick of getting bearing buddys stolen off the trailer in the car park.

 No you dont. Makes no difference.   Bearing buddies fill the hub with grease and keep it pressurized to keep water out if you use them.   If you do regular maintenance there is no need to fill the hub. If you dont then I would fit  bearing buddies.  Biggest thing is to redo the preloading on the bearings a few trips after putting new ones in. Loose or tight bearings lead to water getting into hub.    

Bearings don't like being over packed. Fill the hub to about 1/3 to 1/2 full. An over packed wheel will heat up and with nowhere to go the grease will push past the inner seals or pop the cap.

Pick a grease suitable for trailer bearings and stick with it. If you change greases make sure they are of the same base type. Lithium and calcium greases react with each other.

If you are running bearing buddies, don't over charge them. Allow some of the piston travel to accommodate the grease as it heats up in use.

When knocking in the rear seal plates and bearing caps, clean the surfaces and put a bit of gasket goo on them to seal them against the hub. This will lessen water ingress.

 And if your bearing buddies keep getting pinched, why don't you put your galvinised strips across the bearing buddy? Either way you'll have to remove the strips before greasing. 

Personally whilst I've had trailers with bearing buddies, I've never bought a set. I always repack my bearings every so often & in doing so check the seals. Bearing buddies are a wast of time because all you're doing is mixing new grease with old grease & still not getting it into the bearing cages. 

 bearing buddies are not a waste of time. You need to keep grease pressure up to the rear axle seal, because that is always where you will have your failure. Marine-type seals are designed to have grease sit between the lips to keep the water out, and need to have that prsssure applied. Try it with conventional seals and the innner bearing will fail inside 12 months. having grease com out of the marine seals is not a sign of failure, either, as some would have it, just don't overdo it your braked axle for obvious reasons. I pump in the grease , let it push the caps out against the springs, then fore the caps back to push the grease in, pump in some more. The only bearing failures, if you can call them that, that I have had in a long time were from a seal which didn't seat properly, from  new, and let the water in. Other than that, repak every couple of years. Annual shouldn't be required if you do it right. And putting a strip of metal across the caps to retain them would work if you have a hole in them big enough for the grase gun nozzle--haven't done it myself, but certainly thought about it.

I've never had a bearing failure in nearly 20yrs of owning trailers & 8 yrs of owning boat trailers. I currently own 5 trailers & 2 of them are boat trailers. I always regrease my wheel bearings twice a yr on submersable trailers & pack them by hand with Castrol marine grease. It's messy but it works. I repalce the bearings & seals every 2 yrs regardless just for preventative maintenance. If you have a braked axle that gets hotter or dual axle that has higher load stress then this is even more crucial.  

My point is if you can pressurize the hub to the extent that grease seaps out of the inner seal then unless you pull the hub off, how do you know the seal or bearing cage isn't buggered? I takes a lot of pressure to push grease past TWO caged bearings with a chamber of thick grease in the between. Grease is quite thick. More often than not, by the time it squeezes past the outer bearing & then fills the hub, the pressure it takes to push more of it into the inner bearing & then against the inner seal often causes the bearing buddy to pop off the hub or the hose of the grease gun to pop off the nipple which is when most ppl would think they've filled their hub to the max. 

So if you really want to be thorough pop the hub off, clean & check the seals & in which case you may as well repack the hub by hand which has worked since long before bearing buddies were invented. I've heard so many stories of bearing failure even with bearing buddies because ppl think that squirting grease in there every so often depletes the need to remove the hub every so often & check seals & bearing & cone wear.

The reason bearing buddies work for most is that a lot of ppl don't know how to crrectly pack a bearing & hub by hand & that's what leads to premature wear causing bearing failure.  

Ranmar you are right in a way. The idea behind bearing buddies is to keep the hub pressurized full of grease so that IF a seal fails, water won't get in provided you use marine grease which works in theory but often not as well in reality. The difference between a worn or cracked seal & a completely stuffed seal can be one trip & a completely stuffed seal will let enough water in to wash any type of grease away. That's why it pays to do it the old fashioned way & be thorough. 

Scubafish had a good idea of drilling & tapping the hub for a grease nipple. I'd go 1 step further & specifically do this on the inside of the hub, as far inside of where edge of the bearing cone sits as possible though so you're not creating a weakness in the biggest stress point of the hub. Then fit a bearing buddy for the outer bearing.

But for me, I'm happy to do it the old way. It's time consuming & a horrible job but as an ex mechanic I'm used to it.