how level is level enough?

I’m branching out into the printing realm now and need to know if I’m being too anal about getting my build plate level. I come from an inspection background so we can split hairs if need be. is .003 over a 5 inch span too uneven? Figuring one millimeter is .039 inches I would think I could keep it in the +/- .005 range. Will that cause me any problems? How many people use MOSFETs with their hot beds?

Yes, that is a great number.

I do mine by eye so I am sure I am way worse off that that.

External mosfet, should not be needed unless you are running something larger than usual, or tend to always run at 100C+. I have not needed to take a bed above 65C, but I also use PET instead of ABS.