Guys I have question marks about in the back-end of my rotation, I'll usually sit against potent offenses (NYY, BOS, PHI, TEX, etc.)
If someone has a pretty extreme home/away split, I'll take that into consideration, especially if ballpark factors play a big role (think Chris Young, Padres).
I'm never benching an ace, and I'll usually always start my SP2/3 unless they have been on a brutal streak and are facing a tough lineup.
AJ Burnett against the Sox? Might get bit, but I am throwing him out there 100% of the time.