To be honest I seldom use taxis/uber's but when I do, and am on my own, I sit up front besides the driver.
I do it for a number of reasons but mainly because I find it somehow impolite to the driver if I did't.
I just think that I'm no better than he is (most drivers I've used have been men) and having a chat with them helps pass the time to our destination.
If I'm in a group of people I will sit in the back with them, unless I know I'm the one who is either going to pay the driver or the one giving details of which way to go/best place to pull up, etc.
I understand why most women tend to sit in the back rather than next to a bloke they've never met before but always thought it a bit uncivil of blokes to sit alone in the back.
I guess most people do travel in the back and probably most drivers expect you to as well but sitting up front with the driver just seems right and normal to me.
Where do you sit?