I always glue them in. Time does (usually undesireable) things to wood and plastic, so the nut may not stay in place. Not that the glue is meant to be much of a fastening method - you generally use wood glue, which isn't a good choice for a wood to [other material] joint. But, you don't really want it to be a permanent joint. You just don't want the thing to shift position when playing or fall off when changing strings, while at the same time leaving you able to remove the thing without a lotta heroics for its inevitable replacement. You don't need much - just wet the end of a toothpick with a bit of Titebond II and smear it around in the slot, place the nut and make sure it's centered up, clamp a capo pre-tensioned string nut installation jig on it for an hour or two, and you're safe to file it until the slots are too deep and you have to do it all over again :laughing7: