The smoother you can make the sculpture, the smoother your final product will be.
I use a tool similar to the one on the far left in this set (but mine is made of wood) to smooth the RCT before I cover it with any product. I press it against the surface and pull it along so as to slightly crush the surface.
Before I got that tool, I used to use the back of a spoon to press the surface of the RKT as smooth as possible. The problem with the spoon is that too much pressure will leave a depression in the surface.
After I have it as smooth as I can get it, I cover the sculpture with a very thin layer of candy clay to fill any dips that might be left. I do this because it is not nearly as messy as working with buttercream which never gets hard, or with ganache or melted chocolate or candy melts which will melt from the temperature of my hand and wind up all over the place. I find that the candy clay can be smoothed more easily and with less mess.
The teddy bear was done with buttercream:
The kayak etc. was done with candy melts:
Thomas was done with candy clay:
The easter eggs were also done with candy clay, then covered with gumpaste. They were so smooth that people thought they were real duck eggs: