Longhauling plainly occurs. But in the (approximately )50-75 cab rides I've taken in Vegas over the years, it hasn't happened to me. Not once.
If I'm doing anything right, it's telling the driver how I'd like to make the trip the moment I get in. Like, from the airport to downtown, I'll tell them to take Swenson over to Paradise and then up Las Vegas Blvd. I suspect that, when they think you know your way around town, they figure it's not worth trying to longhaul.
Maybe someday I'll get to make a call to the Taxicab Authority, it sounds kind of empowering. But the damn cabbies just don't longhaul me!