Concerning aircraft, I have, in recent times,become more of a build-first-paint-later modeler. A bit out of convenience, but also from the experience that pre-painted parts (e .g. landing gear legs) face fit and fixing issues. For instance, I pre-assemble cockpit tubs, mont then into a fuselage half, and paint them next. Or I mount landing gear covers and struts, just the wheels are painted separately and generally mounted as one of the final steps. I am not a detail fetishist anymore, though, I rather work for the overall impression than delicate detail that noone will ever see - it's for me not worth the effort (and explains my interstaller building speed at times
)
Similar rule also goes for military vehicles - I try to complete major groups like hull and a turret as far as possible, and paint them later. Wheels frequently separate, though.
Not certain about cars - I have built only a few at all.
Mecha are a different matter - it really depends on the kit. More recent kits (later than 2000, IMHO) have become very clever and convenient constructions - you can build them in separate modules which can be assembled as a final step for the complete model, they also come frequently with more clever and complex joint shapes which allow easy at/detachment of body parts. Early mecha kits are more complicated. They frequently come with simple vinyl caps and feature construction designs that work like a Matushka figure - you have to complete one step (say the lower arm) before you can add something else (upper arm, shoulder next, etc). While you can assemble them and paint later, the joints need some interim PSR and painting, IMHO, a tedious procedure beyond basically questionable part shapes and joint solitions.
