pb and lmp, when I was looking after Wardair's computer systems, I observed frost over the wing tanks from my SLF seat and later sent a query about that to the CP. His reply was that the A310 was certified for that and from what I observed, the takeoff performance did not seem degraded.
As you have both pointed out, frost can be on the underside as well, but I did not do the walkaround
Upper surface airfoil frost likely has more serious performance implications than lower surface and distance from leading edge would also come into play. I saw maybe 10-20 square feet of frosted wing surface, a miniscule fraction of the A310 wing area.
If you see frost on a wing tank, it may well be on the other side too -- a lesson for all of us.