Hi revolverxx,
Either one is okay, though obviously, the 'paint in place' option is much easier. I've done both at one time or another, either to paint the bell end, the transformer lams, or the top plate, or sometimes more than one of these.
If you really want to remove the end bell, you have to, of course, be very careful not to disturb any connections and when replacing the removed components, ensure that they're all back where they belong. It can be a handful, but is doable. Of course, allow some time after shutoff before handling to allow PS caps to discharge.
If painting in place, use care not to get cleaning solvents or abrasive residue into the works, then mask and paint (if you're not planning to just paint all over). Painting in place has the added benefit of enabling easy coating of fastener heads for the transformer screws.