I had a few mandatory P/F courses (1-2 per year) that I took in addition to the standard course load, and my grades in those courses -- or lack thereof -- never came up. If it's a course you really want to take but think you're likely to not do so well, it's not a bad idea to go ahead and take it P/F. I would also double check with your department/university policies on P/F courses and how they're treated in terms of fulfilling various requirements.
EDIT: I should add that my school was very open to P/F courses and even encouraged us to take one each semester, so YMMV. Still, I can't imagine a few scattered Ps in a full transcript can be that harmful -- as always, if someone else has more concrete information, feel free to correct me.