In eq on the flat you should sit, but it's all your preference O/F. I personally do a half-seat but I see others to well sitting also. Whatever is comfortable and looks good, really :]
im riding my one pony in a pony medal this weekend....and actually, in the flat, i do sort of a half seat. her canter on the flat is the worst to sit...but ive gotten top ribbions not sitting in the saddle in big classes. but i just sit up really tall and have my hands up. however if im doing eq on a horse i ist it, because horses...especially eq horses are alot more comfy!
when going over fences, get off your horses back a bit....but sit up. eq judges like to see a half seat while your going over fences, and it shows that you feel for your horse.....sitting on them while they are doing a difficult course cant feel nice for them!