This is a personal preference, but my rule of thumb is to use scissors when you aren't cutting into the hair itself, and use a sharp xacto knife when you are cutting a base with hair on both sides.

The trick to using a knife is to hold the piece in such a manner that there is no tension on the hair. The tension should be on the BASE. Any hair that is in the vicinity of the knife should fall loose and free. If you do it this way, there is very little chance of cutting the hair itself. I even used this method to trim excess from a piece that was ON A PERSONS HEAD!

Scissors will work too, but you have to be really careful not to get those hairs trapped where they will be cut. I just find it much easier to use a knife once you get the feel for holding it correctly.