I have always found that your standard Glock has not has the plastic filed away as nice as possible underneath hte trigger guard. Which requires some judicious sanding just to get that ridge smoothed down. At that point, I'll also usually take down the trigger guard just a bit so my hand fits in a tad better. Not a lot. So no Dremel.
I recall adding a competition trigger to an AR 20 years ago. I was so afraid of messing up the fit. The trigger assembly wouldn't drop in and line up. You needed to file down the bottom to set into your lower. I think I was using 400 or 1,000 grit paper. Probably took me 3 hours of sanding and fitting to get it to fit. LOL