It depends on what you want to do, me I would tape and finish joints, maybe sand the areas to be taped. I have put 1/4 drywall over textured walls that homeowner wanted smooth, 1/2 drywall over popcorn and stippled ceilings rather than tearing out old drywall. The easiest solution would be to tape and finish existing joints.
I'd tape and finish what's there also if the rest of the walls aren't too rough. Problem is, if you get the joints nice and slick and the rest of the walls are rough due to lots of paint, your smooth joints will make the rest look bad or they will stand out. If you can get the same "texture" on the joints when you paint (same "roughness from the roller) you'll be ok. If you add another layer, you'll have issues with the door and/or window jamb thickness to deal with, but that's not a "deal-breaker". It's kind of a matter of how nice of a finish you want. New rock will give you the best job.....
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