Some do and some don't. In terms of knowing about life on other planets of course it would be ideal to maintain planetary quarantine protocols and so on. The problem for exobiologists is that given the absence of visible life in our local system, any life that does exist is fossilized or fragile, and of course we would want to approach it with the same delicacy that we would any other fragile thing.

On the other hand, our solar system is but one among many and the longer we hesitate in exploring this one the longer it will be before we are able to explore others. TAn excess of caution can become an excuse for inaction. If earth is an oasis in a planetary desert, the way to find other life is not by maintaining extreme caution about disturbing the surrounding sand dunes but by trekking across them in search of other oases.