Freeze your subdivision levels, Dynamesh or make whatever changes you want to the mesh, then Un-Freeze your subdivision levels to restore them. Note that Dynamesh doesn’t make quality topology, so you may also want to run your mesh through ZRemesher as well to create higher quality topology suitable for base level geometry.
There are no rules. Generally though, since tools like Dynamesh or Sculptris Pro don’t work on a mesh with subdivision levels, it’s better to use those during the process of mesh creation when you’re making rapid, drastic changes to the form of the mesh. Once your form is established up to about a medium level of detail and the need for drastically changing the form has diminished, some workflows then transition to a multiple subdivision level process for the sculpting of fine detail, UV unwrapping, painting, posing, or texture export. This usually involves a retopologized, low-poly base mesh with clean, quality topology, subdivided as needed to sculpt fine surface detail.
At any point you can project the detail from a high rez mesh onto a mesh with different topology, so you needn’t fear getting locked into one way of working if you decide you need to make a change at some point.