I simply said, which was the truth, that I had a C when I agreed to the P/F. (Now I would have ended up with a B had I not...)
If you do one class that isn't that important, ie philosophy, then it won't be that big of a deal. If they see multiple P/F, then they might look negativity upon that. Think of it this way, there are as many as 100+ people applying for each spot in some schools. It won't look as good with pass/fails.