My general rule is thus:
- Toast and butter very mild flavored ingredients. Egg and cheese sandwich on, say, an onion roll. Salted butter for flavor.
- Toast only is good for firming up bread with looser ingredients. Sloppy joe on a sesame seed bun. The classic triple-decker club sandwich. The toasting brings a little bit of flavor, but mostly this is for structure.
- No toasting or butter is good for the vast majority of other sandwiches, I'd say 85% of all sandwiches you'll encounter in your life. Start here, actually, and add the above if something falls apart or is missing somthing.
Sandwiches. Yum.
