It's ok to tip anyone who performs a service for you. I will generally tip the cable guy, the guy who fixes my garage door, have even tipped the drive thru attendant. It is a way to show appreciation for a job well done. After all, wouldn't you like a tip for what you do?
If they do good service you can. Sometimes i do, sometimes not. They make full min wage for their state tho, im pretty sure, so its not necessary. They arent like regular wait staff who are often only paid half or less of min wage. If i pay in cash i usually let them keep the change if its less than a dollar. When i pay by card i dont leave tips, mostly because using a card means i have no cash and the card machine has no provisiin for tipping.