There's no hard and fast rule... it comes down to personal preference. At this point, holes are drilled, so does it bother you enough to warrant drilling new holes and filling/touching up the current ones?

It doesn't make any difference at this point. You can't move them without leaving a hole that must be dealt with. They're fine; it's pointless to worry about it.

Here's a blog post with different placement options.

The placement is within the norm.

I wouldn't worry about the knobs, they look fine but have the contractor address the crooked cabinet -- door tops do not look level.

I think they are exactly where I would want them placed, not any lower or higher.

It's good. normally they make them even w/the trim piece on the bottom. If you ever decide to change to handles, you'll only have to make a top hole. your cabs are very tall. handles would eat up some of that space for a more proportional look.

The placement is fine. The knob are pretty, but their size is kind of small proportionally to the door.

The hinges of the doors need adjusting by screws inside. Should not be able to see within cabinet. Right door does not look fully closed.