I'm just throwing out an opinion here (and no, I am not an upholsterer ):
A good upholsterer is the difference between a bush league interior job and an interior that looks like Toyota did it.
Generally in my experience whenever you get to the point of wanting to reupholster there is foam work that also needs to be done. Somebody who knows what they are doing makes it look easy and the results are perfect.
All depends on how good you want the final product to be.
Remember: Even Boyd Coddington farmed the upholstery on his rod's out to a pro. ;-)