Never assume things are level and plumb. One or my first home jobs I was doing was putting in a new window in a basement suite bedroom. I had to remove the drywall and reframe the wall to the opening of the window. I framed everything square but when I put it all back together things were not square. I realized the original stud I measured off was not square. I assumed it was and just started to do the job. The window is square and level but the side is not. I had to adjust the blind a bit to fit but it all works.
If I had not assumed everything was square and checked it the window would have been perfect.
Check everything to see if it is level, square and plumb whether it is new or old you just never know.