I've heard that when you're modeling, sculpting a rough draft of your model then retopologizing (making a final draft by placing and connecting vertices) is the most conventional way of modeling. But I'm wondering if it's conventional to use sculpting tools on a retopologized model. Is sculpting only reserved for rough drafts?
This depends a lot on what you plan to do with the model and the type of sculpting you plan on using.
I think what you're thinking of is Dynamic topology sculpting. This is really really good for getting proportions and general form right. The issue with this type, is that it completely triangulates which ruins edge flow, making it bad for animation and texturing. It would be okay to use this for a simple shader only (or procedural) material setup.
The other type of sculpting is Multires. This is best used for detail sculpting, especially when you want to texture, or animate, because it keeps the original edge flow intact. This is probably what you want. To use it, you add a multi-resolution modifier to your mesh (which is basically a specialized subserf modifier), and you sculpt at the level of detail that it adds, then you press the subdivide button to subdivide it again, and you can work on detailing even more at that level, and the process continues. Just remember that you want to do as much detailing as you can at one level before continuing to the next.