Now WWII aircraft, all bets are off, the panels, especially ones with zues fasteners, have at it.
That depends on the aircraft. I've had the opportunity to look at various airframes in various tates of restoration. Many were surprisingly well made and the skins very nicely fitting,butted,overlapped or let in. In the case of fighters many were also taped and/or puttied as part of the effort to achieve a better aerodynamic finish.
It is important to seperate the reality of panel lines on real airframes and how they are represented on models. Modellers are using an artistic technique to try to make their models appear more realistic and less toy like. They are not trying to literally represent the subject they are modelling.
Even the finest engraved panel lines and molded rivets would,if blown up to 1:1 scale,look like trenches and dinner plates!
Personally I am not a fan of overdone lines which make a model look like a patchwork quilt. Subtle shading and washes can make a model appear more "realistic".
Each to their own! It's art,not science
Steve