Geez, why drill a perfectly ggod rim when a valve adapter is so small you can tape one under your saddle? Presta valves hold air so much better, I'd suggest that everybody should change their shrader valves to prestas!
With one of those little adapters, even 'farmer brown' can give you emergency air, so what's the big deal about checking pressure? Fill it until it is firm. If you need to be anal about pressure, buy the presta gauge and leave your rims intact!