Yes it's a common but unsafe assumption to make that walls/ceilings are level and walls plumb/square in any house. Before I hang large mirrors or fit shelves etc. I always set out a line first and then measure up/down from each end of it to compare the distances between the ceiling above and things like mantlepieces or built in furniture below. If these aren't true at least I can then easily work out by how much I might need to adjust my line in order to compensate for what's out. I've often found that eyeing things in works very well, however.
When it comes to levels be careful however. We have a very nice built in leaded light cupboard in our lounge for which I made some large internal shelves many years ago. I spent ages making sure each one was level but when finished couldn't understand why all the shelves appeared to slope down to one side. At first I thought the frame may be out but it wasn't so I then turned to my new 3' long spirit level for an explanation. It had a number of bubbles to facilitate the setting of several different gradients and, to my horror, I discovered that, due to some very unclear markings, I'd been using the wrong one!!
