As noted, the amount of glue is largely dependent on the size of your hinge and the type of material it is being glued into, but the key is to have enough, but not too much. Too much can cause the hinge itself to crack and tear. As the glue is applied, I watch for when the wicking stops, usually 3 to 5 drops. In foam, it will take less than in, say, balsa.

As for lightly sanding the surface of the hinge, the only possible time this may help is in the case of using old floppy discs as hinges. But very fine sandpaper should be used (maybe 400 grit). Deep scratches could actually cause tears over time.