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
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). 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