Well that depends, do you spy on them in other places too?  You know your children better than I do, so in this case I have to tell you to impose the same parenting online that you would in the physical world.  If you would not pick up the receiver to listen to their phone conversations, don’t read their e-mail or IM secretly.  If you would listen, then by all means…
But if you decide not to spy that doesn’t mean you shouldn’t ask questions or even be a little nosy.  You want to know what your kids are up to and who are they talking to.  This is just good old-fashion parenting.