No, I don't tip unseen people.
I tip waitstaff, bartenders, cabbies.....but not mechanics, dentists, toll collectors, fast food counter staff, mail carriers. I sure as Hell don't tip the bathroom dude that waves his hand in front of the towel dispenser.
Too much tipping in America.
I guess if some clerk really did something special for me, but it would be hard to see it got to them properly.