Most of the time I sit, even before transition my whole life the majority of time I sat down. However occasionally I still do, but only at home. Sometimes when I first get up in the morning I pee standing up.
Medically speaking, when you have a penis you void your bladder better by standing up. The long urethra that goes with a penis ,versus the rather short straight urethra a vagina has, just voids more effectively standing up. Try peeing sitting down and then stand up and see if you can get a little more out, usually I can.