Overdoing it is a great idea for practicing them, but you’re right, they can be over used. How much I don’t know…maybe a few times during a song. I use them all the time…recently in Mercy Mercy, and also in Ghost Riders in the Sky so check those song videos out and at least you’ll hear how many times I did it. But I’m just talking about a regular trill….you can also add more embellishment along with trills such as mordents which are when you incorporate the note above and below the main note you’re playing. As for fall offs, I don’t think I can play a tune without having at least 1 or 2 of those! But nobody want to hear every long note you play end with a fall off.