This one's hard to answer without knowing the layout of your bathroom, but basically you have two choices: run the vent parallel to the ceiling joists, or blast through 'em. Do you have the whole ceiling exposed? Is there a way to vent out a different wall by going parallel to the joists? Does it make more sense to do this (ie, location of walls/vent) than to get a hole-cutter and punch through the joists? Also, will you be cutting through 2x6 or better joists? Lots of questions to consider...perhaps some more info would help?