I rode a taxi from my hotel to the airport in Las Vegas. I tipped 15% (I think I was $0.04 cents off)--the tip I left was $1.50. The driver called me a piece of "sh*t" and said that I had to leave a minimum of $2.00.
I know from past experience (and posts in other spots on FT and elsewhere), that it is wise to have lots of small bills when using cabs in LAS, just in case a taxi driver decides to return change based on what he thinks is an appropriate tip.