Bruce writes:
It looks like the fence is only supported by the extension mechanics
and I can imagine a bit of wobble unless the fence ends are somehow
clamped to the table sides. Is this an issue?

While it would seem like a weak setup, if the plate the jig is
attached to is firmly attached to your table it just doesn't move
unless you *really* force it. But on my table, I can clamp the ends
to the table just in case I think I'll be putting some force on it. I
think I've done that once ever.

http://www.delorie.com/wood/projects/router/