There is more than one body to play with, but Mars is among the better chances that we have (and I'd say certainly the easiest chance. Europa is the best chance, but it is not exactly easy to get out there).

Of course we need some sort of cutoff for how cautious we need to be, even if we do embrace caution. Slapping a McMurdo style base down on Mars so that biologists could work more efficiently would risk contaminating the planet, but I think that is a risk we should take (once we have confirmed that there is life there, it would probably be reasonable to loosen controls).