Slightly undersize will make a bolt easier to fit into a corresponding nut or threaded hole, the trade-off being the fit will be a bit sloppy. There are 3 classes of fit, I believe, can't remember the names but it's basically a) flops about like cooked spaghetti, b) not difficult to screw in, but doesn't flop about so much, and c) as tight as a duck's bum - hard to do up but a very close fit.
Also, if you slightly undersize the stock, you'll find it easier to thread using a die. If you single-point it, there's no difference between undersize or full size.