IMO, I think for aircraft like the Lanc, Halifax etc where the wing join is unfaired, ( ie, they just get fitted straight into the cut out groove in the fueslage), it's best to paint them seperately before gluing together - saves alot of tricky masking.
If the joint is faired, it's best to fully assemble and fill them first, then paint. Reason being the paintwork may get damaged in the process , and you'll have to repaint the area, which often looks messier.
For radial engined aircraft, I'd leave the engines off till painting is completed then glue in place afterwards, saves a bit of fiddly handling.