If they're nice guys, they work hard, and do a good job, you should tip them. Anything from 10-100 bucks each, depending on the length, type, and difficulty of the job.
So the place I work, today I got back to "the shop" (central dispatch area) at about 12:20, waiting for my driver to talk to the bosses, then we can leave to go home for the day. Get a call in the office at 12:30, saying one of the drivers had his helper get hurt on a 2 man job, so he needs somebody to come out and help him finish the job. I leave to go help him finish around 12:45, and arrive at the job at around 1:20. (keep in mind I'm paying my own gas there and back, since I'm having to drive myself, not ride in a company truck, so theres about a 4-5 dollar expense) The job is estimated to stop at 2:30, in the original time frame, so that is as long as we are getting paid to work. I log in 1 and a quarter hours working, but the truck took until 330 to unload, as it was full (the other dude hurt himself on the first piece of the unload) and there was a sizable amount of sizable pieces. At the end of the job, I didn't get a tip, so I came out to help finish these people's move, help my buddy out, and I made about...5-7 bucks for 2 hours of hard work. (I'm getting paid 9 bucks an hour) What's done is done, but a customer being thankful doesn't pay off my student loans. I don't mind helping people out either, but I don't know those people, and that driver I've worked with 3 times now, we aren't good friends or anything. (the driver barely makes any money on something like that as it is, so he can't really pay me any more). The lesson here...if the above criteria fit, tip your movers!
The next lesson for you kids, is ALWAYS buy your movers lunch. Here's how it goes: we are getting paid by the hour, and the customer is being charged by the hour. depending on how many guys are on the job, and what time it is, the customer is being charged between 90-200+ dollars an hour for us to move. If its a big job, we have to stop and eat, period. If we stop, buy ourselves lunch, and sit down and eat, lets say that takes us a total of 45 minutes, you just paid 70-150 bucks for us to eat lunch, we paid 5-10 bucks for ourselves to eat lunch, and we are a little salty you weren't lookin out for us (and that we had to spend some of our own money). If you buy us lunch (for 20-40 bucks), and we call in a 30 minute lunch break (so it's not calculated in the final total) you spend 20-40 bucks, and then we are happy you care that we are doing alright moving all your heavy crap, and we happily get back to work. It's cheaper, it's nicer, and it makes the movers happy. and TRUST ME, people who handle all of your stuff, you DO want them to be happy.
I'm always considerate of peoples stuff, but as soon as a customer starts griping, the other dudes aren't so careful with thier stuff...so keep in mind...the happier your movers are, the safer your stuff is (in general), and likely, the cheaper it will be for you.
A final note, if you have boxes that you want the movers to take...TAPE, (don't fold, you turd) the bottom of it. duh!
And tell them ALL that they are taking, so they can plan to pack the truck right.