In most cases you should only edit the proxy mesh, not the smooth mesh. If you edit the smooth mesh, further changes to the proxy mesh can interact with the edits to the smooth mesh to produce strange surface shapes.
The proxy mesh is part of the construction history input for the smooth mesh. All changes to the proxy mesh can affect the smooth mesh. Instead, rotate the group that contains both meshes.
You can move the proxy mesh so it is beside the smooth mesh, letting you see both at once. However, do not rotate the proxy mesh. Rotating the proxy mesh can change the smooth surface in unexpected ways.
Mesh > Combine and Mesh > Booleans operations cannot be performed on a proxy mesh. These operations create new surfaces and can break the connection between the proxy and smooth meshes.
If you need to use these operations, do so before using Subdiv Proxy, or when you no longer need the proxy.
Keep the proxy and smooth meshes connected throughout an animation so there are fewer components to manipulate. (You may want to turn off the visibility of the smooth mesh layer in the Layer Editor).
A high Subdivision Levels attribute can affect Maya's performance. KeepSubdivision Levels low while modeling, and then increase it just before rendering.
Tip: To do this automatically for an object, open the Render Settings window, display the Render Options, and add the following command to your Pre Render MEL code (where X is your object’s Subdiv Proxy node number and Y is the high number of subdivisions, the maximum being 4.):
setAttr polySmoothFaceX.divisions Y;
Add the same command to your Post Render MEL code, but with Y as a low number of subdivisions.
The proxy mesh is renderable. If you don’t want it to appear in rendered images you can hide it (using the ` hotkey), turn off Subdiv Proxy Renderable, delete it, or put it on a hidden render layer.