Graded cards "shouldnt" deteriate over time in storage, so once an 8, always an 8 if you catch on. Where as a raw card always has that chance of becomming damaged through one way or another.
When talking vintage cards, and most likely yours isnt a vintage card (#/5), but a PSA 8 is a VERY good grade for a Bill Russell RC, and would demand quite a premium over the raw version.
I understand that a John Wall RC Auto PSA 8 probably wouldnt be as high in value as a raw one as you have a confirmation that it's not a 10 and most likely won't ever be one, but most certainly when talking older stuff, PSA 8s can be good/great!
I sent some Harris cards off for PSA grading a fair while back, and some came back as 8s. I was happy given the rarity on some of them, knowing that they'll always be that grade, or remain in that condition, as opposed to a raw on in a holder.