Doug, since you're going to eat them anyway, I wouldn't worry about potential urinary problems. Urinary calculi develop when the goats are older. It is a horrible, painful, death for the goat, and there's nothing you can do about it but have them put down.
f you were raising boy goats for other reasons (like I do - for fiber), you would not want to wether them until there were at least 6 months old. This allows time for their urinary tract to develop more fully so that they can pass stones if they develop. Also, it's not a problem at all unless you feed grain to the wethers.
At 6 months old, the best way to wether them would be with a burdizzo. I've been able to band some of mine at 6 months, but it's painful for them.