If you have a sandwich in which the entire sandwich should be heated, there are ways of doing it that I would consider 'least bad' (not necessarily 'best').
I'd heat things up differently based on the bread.
If it's currently hard and/or dry : take a brown paper bag that will fit the sandwich, place the sandwich inside, and tightly seal the bag (fold it down, staple, whatever it takes) but try to leave it a bit of air in the bag (ie, tightly sealed but not tightly wrapped). Wet the outside of the bag with water. Place the bagged sandwich inside a low oven. (or toaster oven, if you have one).
The wet bag will end up steaming the bread some. (it's also a technique to revive stale bread), but will also prevent the bread from burning before the inside has had a chance to warm up.
If it's a soggy, or softer bread : I'd be more likely to put it in a sandwich press (or similar, or use the two-cast-iron skillet to try to crisp up the bread), or put it in an oven uncovered. (in my case, toaster oven, directly on the rack). If using the oven and the bread gets too dried out before the middle is heated sufficiently, switch to the paper bag trick.
There are some sandwiches that just won't heat up well -- anything with lettuce or other vegetables that don't take well to heating up. You might be better off using higher heat so that you heat the bread before the insides have been heated, and just forget about warming it through entirely.
I personally don't like microwaving bread -- it'll seem okay for a minute or so, and then it gets really, really chewy. (and not in a good way ... rubbery)
And, it's not technically an answer to the question, but you also have to consider why you want to heat up the sandwich -- if it's a texture thing, or it's just ice cold, that's a valid reason. If it's a flavor thing (chill mutes many flavors), you can often perk a sandwich back up by adding something hot or sour to it. A splash of hot sauce or some hot pickled peppers will often wake it back up without requiring it to be fully reheated.