Should home inspectors open up "sealed" attic access panels? Heck yes. Every time, unless the owner or buyer specifically says not to. That's my story, always has been, and I'm sticking to it.
Attic access panels are usually not "sealed" anyway; they're just incidentally covered over with some type of ceiling texture as part of the construction process. There are a bazillion things that can go wrong in attics, including new construction attics. Just click this link to see a bunch of photos showing attic defects in new construction homes. Remember, those were all defects on new homes. This should help to underscore the importance of attic inspections, especially on existing homes.
So what's the big to-do about opening "sealed" attic access panels? I've gotta think it's about lack of knowledge. Someone must imagine the home inspector taking a chainsaw to the attic access panel and making a huge mess out of it. The truth is, when done properly, it's nearly impossible to tell that anyone was ever even up there. To show how it ought to be done, I made a short little video showing how we inspect attics without leaving a mess. Enjoy.
Related post from 2009: Break the attic "seal"? Yes, every time.