Every model is a bit different. Generally, you try and assemble as much as you can and then paint the large assemblies as a single unit. With armor, I typically try to assemble the hull and turret so the they can be painted in one session. For the wheels, I would typically leave them on the sprue, paint the rubber tire portion first, mask it, and then paint the the steel portions when I did the hull and turret. If you're unsure, test fit the pieces in question and imagine painting with your air-brush.
Another consideration is how well you can brush paint small parts. Some builders will assemble all the little tools and whatnot on the hull, paint everything with the airbrush, and go back to brush paint the tools. Fro my skillset, I find it easier to paint all the tools separately and carefully glue them to the tank after I've painted it. There's no "correct" way, just whatever way seems easier for you to accomplish.
Andy