As far as I am concerned there is a difference between a skin panel that is part of the aircraft structure and a panel that is removable. A removable panel or access door would, in my opinion, justify a clearly defined panel line. I define a removeable panel as one that can be unscrewed, unfastened or opened for access. Examples would include engine doors, personnel doors, refuel points, panels that require opening/ removing for replenishment/recharging/refilling/rearming etc.
Fixed skin panels are different, on fast jets or airliners, the joins between panels that aren't joggled or overlapped are filled with a flexible sealant and these are painted the same colour as the surrounding surface. It is not unusual to see unpainted sealant though.
Helicopters are generally less aerodynamically efficient as fixed wing aircraft and the joins between skin panels are usually overlapped and riveted together with domed or mushroom headed rivets.
I am a relative newcomer to the plastic modelling scene with regards to the painting techniques that some modellers employ, pre-shading, pin washes etc. But as someone who has been working on aircraft for 25 years, both military and civil, I have to say that the pre-shading fad is just that, a fad. It is unrealistic in my view and I extend that to the over emphasis on panel lines where no emphasis is required.