At around 5 to 7% tbw and had no issues it all depends on the vehicle and what you are towing. Say your trailer is 1.5t then 10% is 150kg = no issues but say your trailer is 3.5t then your now at 350kg which is a lot of extra load on the rear of your tow vehicle plus you might be going over your gvm. So by going to 5% to 7% you reduce that down to 175 to 200kg which = much less load on your vehicle plus less down force at the rear which = more downforce on your steering. it all depends on what you are towing where the centre of gravity is for the trailer, how good the suspension is. Sometimes its what your comfortable with but as far as I'm concerned most of the time 5 to 7% is fine.

Yes. Internet is full of magic numbers and they are probably even true for some specific situations. However, for practical purposes, all we need to know that back loaded trailer has tendency to fishtail. Front loaded less so. Thus, trailer has to be front loaded but not exceeding vertical load capacity of a vehicle's towing coupler specified by manufacturer.
Here is good video illustration :-)

www.youtube.com/watch?v=4jk9H5AB4lM

 dont know too much about the numbers and so forth. if your car can pull the weight without doing a wheelstand your half way there, then you just gotta hope you stop. 

if you look up the vehicle books your towing with it will tell you all the info that is needed including the tow ball weight. 

 Actual testing has shown actual trailer/vehicle stability is more aligned with the actual weight distribution on trailer than the towball weight. I have a link to some testing done some years back, a properly loaded trailer will give best results with the 6-7% mentioned. A lot of blokes swear by the 10%, but only because their old man told them it was gospel. Rearward weight distribution on the trailer is the killer for stability, apparently.

I always run around that, never had a problem.

You're on the money there Paw. 5-10% is great. This is why towball downforce weights are usually 10% of towing capacity. 
If you balance it out less than 10% it won't be an issue until you go below 5% from my experience and you may start to get some sway downhills, around corners or at higher speeds. 

I've done a bit of trialling on mine with putting a 250hp on the back which took some weight off my towball unknowingly to about 3-4%, which resulted in some sway at times. Moving the boat forward a couple of inches helped more over the towball and balanced it out nicely. Roughly now around 7% and poses no more sway or inbalances. Going up to 10% is uncessary i think and will cause more uncessary wear on your car suspension. 
As comments above, good trailer suspesion will counter act some sway, and lower COG, but ideally balancing correctly is cruicial. 

 Thanks Gents. Think I am going to drop my towball weight as at present I will probably be over 10%. 

My boat tows better at 10% = 320kgs!

 Before I updated my trailer I had over 300kgs on the ball, towed beautifully. When I purchased a new ally trailer I had 180 kgs on the ball. Tows perfect, Probs better than the previous despite less weight on the ball. I put this down to the length of the actual hitch which is a metre longer than the previous.

I am thinking that the longer the hitch the greater the leverage generated so therefore less weight on the ball. Dunno if this is right but it’s all I can come up with.