I don't make my own lams, but I find that it is useful to verify lam and glass thickness even when I ordered from the good guys. I also find them invaluable for measuring the final bow or measuring a bow I want to copy. Once you have them, you'll find a way to use them.
As for which ones to choose, it doesn't really matter for the precision needed for bow making. The real difference can be how well they hold zero, how smoothly they adjust, lock down, etc. If you get a dial set, I've never had a problem with the $20 ones. The cheaper electronic ones can sometimes start to give erroneous readings when the battery is low, but if you check zero every so often you're usually safe. I doubt you will need it NIST traceable like some of what we do in the labs and machine shops here.