I've certainly never been stopped from walking into 'lower' cabins, and have never dared to venture into a cabin ahead of me (normally F!).

On my first (and currently sadly only) F flight last summer I did ask one of the F crew if it was OK to go for a wander, and he said that was fine, just to mind the rope

I do feel a little awkward sometimes walking into whY, going to the back, turning round and then walking back up front through the curtain. And when you do go back 'there' on a longhaul, you really appreciate that comfy CW seat!