Agree with falconred, disagree with sfo, if they provide extra personable and extra fast service that's worth rewarding/encouraging/thanking monetarily.

Whether or not it's hard or their job or a benefit to them somehow is beside the point.

And if you're going all the time and can encourage an extra positive experience, and the tip is reciprocated in extra good service, great ^

What I tend to object to is tipping for nothing/that doesn't get me anywhere compared to not tipping.