Nothing makes a tow guys day like a tip, definitely not expected but is definitely appreciated. $20 is fine. In the old days when I had a AAA contract when I would get out in a truck I would crank the tips and get the guys kicking butt trying to beat me.
Tips are good if a driver does an exceptionally good job or goes above and beyond what should be expected. I agree with the previous post on amounts. The tow itself is a service contract, that contract includes a certain level of service that should be expected as part of the contract. In my opinion tipping a tow truck driver should not be expected or even required. It's not the same as sitting down and having someone serve you.
I bought my tow truck driver lunch and tipped him $60. AAA tow from RSM to Holtville. 197 miles. My brother tipped a tt driver and the driver gave him his number and said he'll hook him up in the future. Aka to drunk to drive.