The best idea is to put all the telltales that you think you will need anywhere you want. Then go out and sail. Try to get as many working as you can. There will be some that you cannot get to work with the others no matter how hard you try. At the end of the day take the ones off that did not work and keep all the others on the sail. This way you have the tell tails you need without worrying about the extrainious ones that do not work.

I usually use the tell tails up high to see if I am sheeting the main tight enough downwind. Most of the time if I have to ask, or look, I am not sheeted tight enough.

Later,
Dan