Always tip as much as you can afford. If you can't afford to tip at Subway, don't tip. If that extra $5 or whatever really is nothing to you, give it. It's really not that hard.
Same at a restaurant. If money is tight, tip what you can. If you have plenty, give a lot. Always assuming polite service. Put yourself in the server's shoes. I'm guessing most on this board are older and have been working awhile and make ok money. Be generous if you can.