It is NOT wise to take any kind of antibiotic when it is not needed! Somebody wiser than I can probably better explain why, but it builds up resistance in your system - geez, I am not so good at explaining this. Somebody help!
Taking an antibiotic when it is not needed is more harmful than helpful. Please don't do it.
Actually, the biggest problem in taking an antibiotic when you don't need to is that it will destroy the NECESSARY bacteria in your gastrointestinal tract--making digestion much harder, and opening yourself up to opportunistic GI tract infections when you go off the antibiotic.