Of course you _can_ do that, but personally I don't see the point in it. You should only sign a contract if you have committed yourself to show up and work for the company - they've done the same, after all, and it causes them significant effort (and cost) if you flunk out later on, or even worse, if you show up your first day to hand in your notice. Be fair.
You can try to avoid this situation simply by applying to the firms you consider as possible employers at the same time, thus increasing the chance that you will be able to have multiple offers on the table before you put your signature under one.

For a more thorough view on the situation, please see my post called "You got the offer, now what?" on the KC: http://www.killerconsultant.com/consulting101/you-got-the-offer-now-what/