In general, I agree with Mel Progson. I go to see these bands to see them play their own songs.
In this case though I would not be unreceptive to a few tastefully placed Dio covers. I pretty much expect it from Hammerfall. They strike me as the #1 most likely to cover a tune. I also would not be too surprised to hear one from Blackguard, Nocturnal Rites, or Stormwarrior.
I'd be fine if bands choose not to do an actual cover but say a few words, or dedicate one of their own songs to Dio (to me that's a classy and tasteful way to give respect to a departed).