If you're driving a pickemup truck on a Texas state highway -Interstate raceways don't count- the wave is about as casual as it can get... You've got one hand laid across the wheel at the 12 o'clock position. You'll acknowledge passing another truck by casually lifting only the index finger. Best to be wearing your cowboy or feed store gimme cap. Yes, I have a faithful Ford truck and the hats to go with it.
Certain Drive Texas Friendly customs do transfer to the motorcycling crowd. The bike wave is executed with the left arm down at about a 45-degree angle and the first two fingers are extended. Here's the deal on the two fingers thing: You're both saying, Keep two wheels down and the shiny side up. Newbies wave with all fingers extended. I see that with a lot with the mid-life HD lads.
In my riding, I would estimate approx 85-90% of the cruiser folks hit me with the wave. Maybe half that percentage with sport bikes. Often the sporty boys are too new / too cool to have matured from the squid stage into true motorcyclists. If two-up, it's often the female pillion that waves. Also, should any rider be making a turn or maneuvering in traffic, there's no hurt feelings if the wave is not offered. Sometimes, you've gotta keep both hands on the bars and your head on a swivel.
Now all y'all drive safe and friendly. That's the Cowboy Way.