For the last few years binding has been sewn and the results are pretty fantastic.

Fantastic Four and Spider-Man came out in 2005 and 2007 respectively, which means they were glued. But they also recieved second printings which had sewn bindings. To make things trickier, the covers for the first and second edition are the same, so its mostly a crapshoot. There is a way to tell if a FF omnibus is sewn or glued. If its glued then the Marvel logo on the side is a white box with red type, if its sewn then its a red box with white type. Unfortunately Spider-Man has no such giveaway, which its stressful as it is one of the thickest omnibuses and thus kind of needs sewn binding.

I know that everything from Iron Man onwards is sewn (the omnibus that came before Iron Man is the Daredevil companion, which is glued). So your Iron Man and Avengers omnibuses are going to easy and safe to read.