Aircraft Build Progress Prior to Priming

Attach everything you possibly can before priming. That's my motto. Others have a different motto. Whatever parts get glued on after prime and paint must be attached with superglue (or white glue for canopies). If you want to join a part with regular model cement, you best attach it before prime and paint. Example-109 landing gear can be tricky to align, thus it should be glued on with regular cement for a good bond and to give you time to align it. Canopy parts that will be in the closed position, as well as windscreens, should be glued on before prime because they sometimes require filling and sanding where the clear part meets the fuselage. There's many simple ways to mask and paint gear legs and wheel bays so don't be afraid to attach them before you paint the model.

This is a very frequent topic in this forum, so you might look back through past threads.

I personally say there is no general rule, it depends on the particular kit. I always go through the instructions and map out a plan.  The most general thing to say is that most kits involve building assemblies, then assembling the assemblies into a complete model.  Some parts and some assemblies need to be primed and painted before final assembly, but there is always painting after assembly too.  How much and when to do each depends on the specific kit.