I would advise you not to paint while the parts are still on the sprues. The reason for that is that as you must have noticed, plasic parts are created in a two-piece mold that leaves, even on the best enginered kits, a visible 'mold line'. This needs to be carefuly removed, with the help of knives, sand-paper and/or files. Leaving the 'mold line' will result in it becoming VERY apparent once you reach the painting (washes and drybrushing in particular) and weathering stages.
I do things in stages, hull and inside if there's any details included in the kit first, adding wheels and tracks, painting the inside first, gbuilding the turret at the same time, but as a different kit. Next come a coat of primer, followed by the color(s) for the camoflage. Detail painting comes next with the rubber rims of the wheels, the tracks,.. Then add all the pre-painted tools, accessories,.. Then washes and drybrushing. Then final assembly (turret + hull), then weathering.
Indeed, many small parts can be painted before they are actualy glued in place over a painted model (open hatches, tools, tracks, wheels,...). The key is making sure that what you do with those parts will not 'look strange'. For instance, do your weathering once everything is on. Dust on the wheels and none on the turret will look weird! Also, be careful how you glue those little parts on the model: liquid cement will attack some paints (enamels for instance), superglue can 'fog' adjacent areas. PVA (wood) glue is a safer bet, 'cause it can easily be cleaned off, but the tack is obviously lower and manipulating your model can knock things off...
I'm not really answering your question, possibly because I'm doing different things to different models, but I think that if you follow some of the advice above, you'll end up with better results.