What Simon said: try a few methods and see which you prefer. Speaking for myself, I like to leave wheels and/or bogies separate as far as possible because I find it easier to paint them that way. You probably have three options: build the entire bogies and add them to the model before painting, build the bogies but attach them after painting, or see if you can leave the wheels out of the bogies and install them after painting (with the bogies already on the tank). That last one may be difficult or impossible, depending on the bogie design, though.