Bob---------------This sounds like a multiple choice test where # 5 ( all of the above ) is the correct answer. I'm assuming you mean on existing homes and not new builds. From my experience a gasket is always on the list and the other three depend on the gap around the cut out. I always use caps on recepticals too. Foam always makes a good backer for caulk or mud.
Yes existing home, if it was new construction I'd have the walls foamed and be done with it. Gaps are typically uneven between the drywall and electrical boxes, one side may be tight while the other has the switch plate barely covering the gap. Boxes don't line up with drywall, some are too tall, some to low. Oddly once the switch plates are on everythign looks OK. Typical construction for a 12yr old house around here.
foam gaskets are little to no use, try a blower door test and test the results you will see little to no difference, the air comes thoruogh the electrical holes in the sockets and from around or through the box. www.efi.org i believe is the correct website sells electric plates that seal or have a sliding door, this is good, but i have found by testing that the baby protectors work great and are cheap, try a blower door test and see. Caulk or foam the box to the drywall works and is easy and cheap, pulling out the electrical outlet to seal the back of the box is not cheap or easy and there is liability. go for the easy fixes first and test and measure the results. These are usually the small leakage points anyway, want to really reduce infiltration, look to the big areas, penetrations to crawl, attic, attic access, interior wall plates in the attic, large openings-coffered ceilings, drop downs, chases, chimneys, doors and windows, ducts, you probably have heard of the areas to look. hope this helps.
My concern is the temperature difference between the outlet and wall is about 8 degrees, even w/o blower door/wind involved. Baby protectors are already in place for small children in the home. I'm deciding weather to go with caulk or foam, my concern is it going past the gap and just filling the wall behind it. I'm not going bother will pulling outlets to seal the wires behind, too much effort for the return.
if that is your concern then foam would be the only alternative. if you can insert the tube and foam seal arouind and behind the box you might be able to air seal it and insulate it resulting in better temperature and infiltration. Some foams are better than others if indoor air quality is a concern, just like cualks. I doubt you will see the differnece in your utility costs, but if you are doing the work, what the hey might as well make it the best you can. You could throw in the foam gasket also if you have them, not that they will make any differnece, some guy is sitting in jamaca right now laughing at all the utility companies and weatherization programs that bought his foam gasket, LOL! Best of luck, would like to know the results, temperature and blower door numbers?