I don't know what the rule is but if they do a good job, I tip about $10-20/guy, depending on how much they deliver and the stairs involved.
Once I had furniture guys deliver an entertainment center in pieces and they left it that way, in pieces! They were gone by the time I realized they hadn't put the top on or screwed the back in place...don't ask. They grumbled the whole time about delivering a heavy side table. They didn't get a tip but every other delivery team I ever had did.