I accepted on the spot. I had interviewed with a Vault100 firm that I really liked and knew that I wanted the job, so as soon as they offered it, I accepted it. I also cancelled all of my other callbacks. Honestly, I'm breathing a sigh of relief. I get to sit back and concentrate on (and enjoy) my classes while most of my friends aren't doing their callbacks until October.
If you are unsure, keep your offers open, but if not, I would say to just go for it. If you do accept and you still have other callbacks, cancel them. The firms you reject will appreciate that you aren't wasting their time and resources (and you also won't be wasting your own).