If it's dovetail joint fitting tight, lower the bit a hair. Highthen to tighten, lower to loosen. Should be that simple! Do the test cuts/setup first!
Box joints? Whole 'nuther issue. An undersized bit there will get you pretty tight joints.
I have good luck with my Whiteside bit ... not so much with an MLCS bit. The MLCS 3/8 bit I have is about 5-6 thousandths under size. That why I bought the Whiteside bit.. it is within +- 0.0015.
I have found that hand tools are the best choice when I want to make mistakes at a slower rate of speed.
I don't suffer from insanity. I enjoy every minute of it.