Like anything it’s conditional.
The kart is designed to work with the driver transferring weight into the top of the seat, through the seat struts, down to the bearing hangers to lift and plant the rear wheels. So a firm posture, keeping yourself planted in the seat and pushing and pulling on those seat strut connection points with your torso is going to work the kart most effectively.
That’s not to say you can’t be fast with a different posture though. You might set your kart up a little differently if you lean differently in the seat and achieve similar results. As discussed, you can change the kart’s handling pretty drastically with the way your shift and move your own body weight in the kart. So if the kart isn’t going planting the outside rear enough, you can lean further to the outside of the seat and try to put more pressure on it. If the kart is hopping or overloading the outside tire, maybe you try slouching a bit in the seat to lower the center of gravity, or you lean inward in the corner a bit to keep some pressure off the outside rear.
I tend to keep myself as upright, firm, and leaning outwards as much as possible. The most important thing is keeping some consistent posture. If you’re flopping all over the place, the kart will react to that every time and you’re going to have inconsistent handling issues.
Big topic about posture here: Posture, and head movement
Posture was another one on the list of things I wanted to make a video on.