Yeah, we are allowed to open anybody's cadaver and take a peek. I don't know about going ahead and dissecting another group's cadaver, but as long as you leave everything where it was when you opened the bag, it's all good. In fact, you NEED to look at everyone's body because you're obviously going to be tested on everyone's body and not just yours.
For instance, this Monday was our first anatomy written and lab practical. Friday-Sunday there were people in the labs constantly looking at everyone's cadavers. During our scheduled lab hours in the afternoon, we mainly dissect our own cadavers. But of course there can't be all five or six of us cutting and slicing away at once, so usually there will be three or four of us dissecting, while the others go and wander around the other cadavers or go listen to the professors talking about the prosected parts they have brought out for that lab.
Overall, I love lab... much more than lecture. Although, I must say I learn the anatomy most by reading and looking at atlases, then I go to the lab and put everything together.