Well, the answer to that is yes and no. Yes I have to if I use the cheat to "preview" what he is sculpting (if I leave him alone and let him finish it, then there are no problems). No, I don't if I don't use it.
I'm not exactly sure why he won't finish it. I think it has something to do with it already being "created" without actually going through the process. I use it to get new items like the chess set, toilet, etc. I just have him start a sculpture and delete the big block, leaving what he will finish there. If it's not what I want, I just scrap it and start a new one.
Once I was working with metal and my Sim caught himself on fire so he stopped sculpting (I had already exposed it and he was making a new chair) and I had him jump in the shower, but when I clicked on the option to finish sculpture he just stood there like he couldn't reach it or something.
I think it has something to do with stopping after exposing the sculpture? I mean, sure I can get him to start and stop so long as I don't use the cheat.