If the fork is actually ground asymmetrical, then grinding it symmetrical is unlikely to significantly weaken it assuming good weld penetration for a welded joint.
Just pay attention to what you're grinding on.
But it all depends on the construction. If the parts began asymmetrical, and the the weld followed an incorrect line, then you could damage it a bit. Don't thin the fork blades.
There is often more material in a weld, cracks and stresses often form next to the weld, rather than in the weld.