On small boards, you can usually rest the heel of your hand on something to steady it. Or, maybe your forearm. I agree, on larger boards, you don't have a place to rest your arm without touching the board, so that is a problem. I'd invest in good tweezers and a stereo zoom microscope first, WAY more useful. I do have a full-auto P&P machine, but did a LOT of manual P&P before I got it, and still make prototypes by hand, as well as a lot of rework. I've always found ways to just use my hands and good curved-nose tweezers to do it.
Jon