Lip sync only for me. My singing voice is not appropriate to use. However, I lip sync pretty well, so I still look like I'm holding a concert in my car.
I do a little of both. Although my sense of pitch is good, my range is limited. This leads me to mix things up, when the vocals go to high or too low to sing along with. Often, I find myself singing what would be the higher notes on the bass clef. You can blame years of playing trombone for that one.