Well, all the cool kids seem to tune without a body in the seat so there's that (sarcasm). I'm not one of the cool kids so I tune with my stocky body in the seat (sarcasm again but everything after this is sincere). I've never measured with and without me in the seat but there's no way a human body ISN'T throwing things off at least a little. Maybe it's not such a big deal if you're running dedicated midbass speakers with midrange and tweets up high? Even when I had a back seat I tuned sitting in the drivers seat and it works for me. I'm sure someone could put their fancy 5 mic tuning rig in there with fancy software and blow my mind with a truly professional tune but don't have a friend who's a tuning prodigy in easy reach and not gonna pay someone to tune something that's just a casual hobby for me. Maybe if I took this more seriously I'd get a professional tune from someone with a good track record just to compare it against my own that's already been said to be pretty damn good for being done by someone with questionable hearing (up for debate and it varies depending on allergies and if I remembered to flush out earwax BEFORE doing the by ear stuff).
EDIT: If I can get off the lake long enough while on vacation and the dogs are quiet (preferably in the house sleeping) I might do a new tune and share it here. Maybe I will, maybe I won't, lol.