PJR is right. For the game of skeet, you want your chokes about as open as you can get them, since the shots are relatively close. For sporting clays, it all depends upon the course. If I'm shooting an unknown course, I'll either ask in the clubhouse, or check with the trapper to see what they think most folks use.
Typically, IC is the best bet, although I've run into a couple of courses where the difficulty is provided primarily through longer shots. On those, Modified may be a better choice. I've yet to see one which requires much more than that.
In any event, I try to avoid the "change chokes every station" mentality that some gamers use. I've shot with people who do it... and I don't think a single one of them has outshot me. What I will do is use a couple different types of ammo. For example, I may use #8.5 shot for most targets, and reserve some #7.5 for longer shots.