The first thing to remember is that curbside skycaps are independent contractors and not airline employees. That's why they can accept tips, and expect them, while agents at the check-in counter inside the terminal are actual airline employees.
If you don't want to tip to have your bags handled at the airport, all you have to do is walk inside and hand them off to the counter agents. It's that simple.
However, in Las Vegas, especially on Sunday and Monday mornings, or after a big convention or event, the lines at the check-in counters can be insanely long, while the lines at curbside check-in, though long, aren't halfway to the moon. It's often worth the tip to take care of your bags outside, then walk in unencumbered and free to bypass the hordes.
Outdoor baggage handlers often greet you right at the drop-off vehicle with a baggage cart. Since their income is heavily dependent on tips, they have to hustle. If you give your luggage over to one, he'll ask you for your ticket or boarding pass and ID, print your baggage tickets, slap the bar-coded label onto your luggage, and hand you your claim ticket.
The typical tip is $2 for the first bag and $1 for each additional bag, unless they're oversized, which get the $2.
If it's raining, intensely windy, or over 90 degrees and you get good service, you can tip more. If all you have is a $5, hand it over (and don't, for crying out loud, ask for change. This is Vegas).