If parts of the tank open up (ie hatches), and there is internal detail (ie new rhino tank), then it is worth the effort of painting the inside of the visible parts of the tank. If there are no hatches, then it isn't worth painting the inside, you can't see it; and if you can see inside, but there is no detail, then a very basic basecoat is all you'd need.
As far as painting the detailed internals of a tank (ie new rhino) then you are better off painting then assembling so that you can actually see what you are painting.
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