In all the years I've been working in the remodeling and home repair industry, as a Memphis handyman, kitchen installation specialist in Denver, and hardwood flooring tech in Phoenix, I've yet to find a house that is perfectly level, square and plumb. There is always some variation in the level of a floor, the width of a room, or in a wall that leans in or out.
A house does not need to be perfectly level, square or plumb. Most of the variations you run across, in even the most expensive custom homes, aren't visible to the average homeowner. In fact with an average deviation, such as a floor which is an inch wider at one end, or that dips half an inch in the middle, or a wall that leans back half an inch, most tradesmen wouldn't notice it unless they pull out a tape measure or a level.
To build a house which is perfectly level, square and plumb would be prohibitively expensive. It would just take too much time and effort to try to get it perfect, and would be a wasted effort as who would notice it, but another builder with a fanatical eye for detail?
What can be surprising as a craftsman, however, is to walk into a house that has a drastic sag in the floor, walls that seem to follow their own paths, or lean like a drunken sailor. You step back and go "Whoa! What happened here ?!", and the homeowner gives you a blank stare. They have no idea what you're looking at. It is surprising what details become glaringly obvious to you when you work in the trades, that are completely unnoticed by a majority of people.
Now why is this important?
Because it isn't an excuse for workmanship that SHOULD be level, square and plumb.
A floor installer must level a floor to industry specs for the material he is installing, such as tile, wood or laminate flooring.
A cabinet installer must level and plumb all the cabinets, particularly if the counter-top will be granite.
Or joints on crown molding or base molding, the list goes on. The point is no house is perfect, some are quite far from it.
If you have work done in your home, and you see that it isn't done correctly, if you bring it up to the installer and they claim "Well, your house isn't square", "Your floor isn't level" or "Your walls aren't plumb", this isn't an acceptable excuse. They never are level, square and plumb. It's the craftsman's job to work with the job site conditions and make his work level, square and plumb despite this.
Now granted, the deviations can be so extreme that it requires a lot of extra work to correct for it. But so what? They should be explaining this to you, or the estimator should have caught this, and then explain that there is an additional cost for making it right. It then becomes your decision whether to correct for it or not.
It should not be the homeowner's lot to have to accept shoddy workmanship, with the excuse that "It's your house's fault!"