Looking back at previous games I've ran, I know I've done both and I guess environment has something to do with it. When one group I play with meets at a friend's place, one guy usually stands when he's the DM. His minis and gear are at the kitchen counter top, higher than the table where the mat and map are and we sit or stand around the table.
With the group that prompted this thread, it's like I'm forced to sit down. The "DM's chair" is at the head of the dinner table. Flanked on one side by a small table holding napkins and an Elmo toy, the other by a bookcase and boxes miniatures. You pretty much gotta sit in the chair. Unfortunately, the chair doesn't move well, having wheels, but sitting on a plush carpet. I've been getting annoyed at being "trapped" the last few weeks. Throw in his new kid crawling along the floor and something had to change.