You can sculpt and paint a posed model by turning on Transform >> Activate Symmetry and then Use Posable Symmetry. This will only work if the model’s topology is symmetrical.
If you are sculpting a ZSphere preview, be sure to store a morph target before turning on the preview. What this does is instruct ZBrush to calculate the topology and then continue to use that topology every time the preview is turned on. If you don’t do this, then when you re-pose your ZSpheres after having sculpted and/or painted a new topology might be calculated. That in turn would mess up the work you’ve done. ZBrush will usually keep the same topology for each preview during the same session, but when you come back to your work in a new session you would almost certainly have problems. Storing the morph target before turning on the preview and starting sculpting/painting will prevent that.
By the way, you cannot add or remove ZSpheres once you have started sculpting/painting. That will automatically force a new calculation despite any stored morph target.
As for the basics of your question, there is no right or wrong way. You can do either, depending on your own preferences and the needs of your particular project.