You do not need them. As in it will not cause any errors.
In oblivion there was a single head mesh for all races and sixes(cept the freaks
). In F3 there are several heads, while the facegen parameters are fairly bland in comparision to oblivions diveristy, you may still have a fat head now and again that causes some clipping. An egm may help minimize that. But even with an egm you may run into a clipping issue anyway. especialy around the cheeks and face. In whitch case you might want to fiddle with the mesh and reexport and do a new egm. Its just a topology issue in those cases- the mesh geometry isn't near enough to the head meshes, which the egms will be based off of. and moving a couple verts a tiny fraction has solved this for me.
The only time I wouldn't bother making one is if the helmet completely covers the head and face. Then there really is no point and will probably be much better for performance and mem usuage- If the mesh starts the hits a couple thousand polygon mark, egms end up being pretty large in file size. Several times the size of the mesh itself.
It takes only five minutes to do a batch of egms. I spent more time writing that post than it does making some