I knew of one GI who did it in later part of his career. no strict rule one has to stand. you can practice during job for few cases and see how that works out. ergonomically everything in scope room is set for stand and scope, so you might end up with neck sprain and neck injury
I know a few people who sit, but for specific situations, not because they prefer to sit.
You shouldn't be worried about standing all day, in fact, if you are in the field of GI, it is a blessing that we have the opportunity to stand all day and not be sedentary like some other office-based fields.