Yes, it is, and we've been tipping skycaps for a lot longer than 10+ years.

IMO, it's incumbent on worldwide travelers to learn about the basic customs, traditions and mores of the places they visit. When I visit other countries, I've already learned some basics about things like tipping. Apparently, not everyone agrees with me.

Here's a basic rule: Whenever someone handles/touches/carries my suitcases in the USA, I tip them unless they're behind the ticket counter (inside) at an airport. Skycaps, cab drivers, hotel bellhops, etc.