So if one looks at a guitarist from above, and takes one line through their shoulders, and another line along the direction that their guitar’s neck is pointing, let’s define the smallest angle between these lines as Θ.
Is that right? 
I’ve seen many players with a “right leg” style posture, such as Guthrie Trapp, who can have a very pronounced “Θ” angle, possibly even more than 45 degrees, and even retain a large “Θ” angle when standing. I’ve wondered if the lack of forearm contour on a traditional telecaster body can be a factor in influencing this sort of high “Θ” posture.
But I think torso/abdomen geometry may sometimes be a factor as well. When I was much younger and thinner, I would practice seated with a strat-shaped guitar on my right leg and have a “Θ” angle very close to 0 degrees.
That said, I do think “classical” style is more likely to result in “Θ” near zero than “right leg” style is.
Yeah I’m starting to suspect that most people hold their guitars at Θ = 45 or so (when viewed from above) and that they might keep a roughly similar angle sitting or standing, so that their most convent way to sit is with it on their right leg. But the classical people sit “classical” style and they seem to have Θ = 0. Now, I wonder about the flamenco people, as they sit with their guitars on their right leg? Guitars are interesting because there are a zillion ways to hold them, bend one’s wrists, etc.
Honestly the easiest way to have a 45 degree when seated is to just… wear a guitar strap while playing guitar seated.
Sorry, my writing was unclear, I didn’t mean 45 degrees in the sense of how much the neck points towards the ceiling, but if it points left (0 degrees) or, say, straight ahead (90 degrees).
I’m sure there’s some variation here - I’m right leg, and checking the few pictures I have on my phone, I’d say I’m more like 30* than 45.
So, is there a preferred angle for this? I usually rest my guitar on the right leg, and I’d guess my angle is around 30 degrees. However, I’ve been having some shoulder pain lately, and it seems like a larger angle reduces tension in my right shoulder but might not be as easy on the left shoulder and arm. When I put the guitar in the classical position it feels even more relaxed for my right shoulder, but I’m not used to it so it feels awkward for my left hand.