Rhyan, you ask the most interesting and yet sometimes toughest questions.
Fills in general are about bringing a phrase to a conclusion or pre-empting the next phrase. So you tend to use them at the end of 8 or 16 bars if you are using the rule of powers of 2 in your structures. try not to use exactly the same fill each time, as it will become uninteresting very quickly.
It's also good to use variations in snare and kick patterns within these smaller structural units, simply to keep interest alive. Not wholesale changes but additional drops or leave it out for a bar anything that keeps the punters guessing as to what is gonna happen next.
I have some other thoughts, but might post them as a PM as any reference to theory around here seems to get stomped on at the moment....
