Getting your hinges centered on both surfaces matters somewhat, but being off is no great disaster as long as the surface moves without binding or snapping. The more I hinge, the more careful I try to be about putting everything neatly on the center line, getting the hinge pins (on pinned hinges) all on the same axis. I like the GP/Dubro/Klett pin type hinges, but on some installations I like the Robart hinge points better, and on most ARFs I substitute Radio South CA hinges for whatever is in the kit. I tried the GP hinge slotter, wasn't thrilled with it, sold it after using it once. I seem to do well with the Dubro hinge cutting tool, the fork job with the picking tool, used for pin type hinges. I start the hinging process with a ball point pen line straight down the middle of each of the facing edges. I bevel down to that line, then cut carefully, trying to stay perfectly on that line. It's never perfect, but it helps to come close.