Unfortunately the Mk.I Eyeball and Mk.II Camera Lens have a way of peering into crevices that are otherwise inaccessible to a brush or airbrush, so over-assembly can potentially create issues later in the build. The undercarriage is the area most likely to be missed.
It’s very common to see modelers painting and weathering undercarriage before bogies, road wheels or other suspension items are attached. Suspension and road wheels are also painted separately and then installed over mudded and weathered undercarriage. Tracks are installed over suspension and road wheels and then final mudding and weathering applied
Probably the best overall routine I’ve seen lately comes from @stikpusher, who will pretty much builds, paint and weather everything below the mud guards in the manner described above, and doing it all before the upper/lower hulls are mated. This prolongs ease of access and ease of workability when it comes to installing the tracks. He’ll then mask off the undercarriage and then proceed with painting everything above the mudguards.
Maybe that’s why he’s called @stikpusher because of all the sticky notes he pushes into his undercarriage to mask it off 
Totally trying this on my next build because it just makes so much sense regardless of the style of track you’re working with
Anyway he very kindly described his process here:
Prepainting other items before assembly such as tools, racks, external fuel cans and stowage are really down to your confidence levels on getting complete and precise paint coverage from prying sightlines. There’s certainly no shame in painting separately and then fixing with CA glue later, especially if it means not losing your sense of immersion in the kit over some misplaced colour at an interface of parts.