Yes, I did put trim stripes on the sail. I used the top mast as a straight end and drew them on with a magic marker.
Yes, it is easier to see the camber and where the draft is when you are playing with the controls.
Let me warn you that it is probably illegal-- particularly if you go faster after you put them on.
Anything in the rules that is allowed is allowed. If the rules don't say then it probably not allowed. If you do things that make you slow--nobody cares. If someone thinks this makes you faster then it will be illegal..
Probably no one will care, if I have them on-- we will see.
If you want to try it, put the strip on with plastic electrical tape and then when they protest you can peel it off.