As a practical matter, I don't think numbers should be unretired (at least until some theoretical future Numbers Crisis)... it's a bad PR move, it's disrespectful, it's unnecessary.
That said, the Celtics really have retired too many. They've gotten a bit crazy over the years. If, theoretically, we could start over and put up only the numbers that really need to be in the rafters without offending anybody, here's how I see it:
We should take down #'s 1 and 2, which are retired for Walter Brown and Red Auerbach, and give some other tribute to those guys. Retiring numbers people never even wore for the team is silly.
We should take down #3. DJ was awesome, but he was at least as good for the Sonics as he was for us, and we only got the last chunk of his career. A hundred fine NBA players have done as much for their teams without being retired.
#6 should stay retired. That's Bill Russell. No-brainer.
#10 is Jo Jo White. That's a tough one, but I'm fine with leaving it retired until the Number Zombies devour it at numberpocalypse.
#14 is Cousy. Leave it retired. Easy.
#15 is Tom Heinsohn. As a player, I'd probably unretire it, but adding in his contributions as a coach and broadcaster, I'd say leave it up there.
I'd probably unretire #16, Satch Sanders, but I'm open to arguments from scholars of that era who disagree.
#17 is Havlicek. It should stay up there.
#18 is Cowens. Leader of two championship teams. Leave it up.
I'd take down #19, Don Nelson. Pretty good coach, pretty good player, not really a star.
I'd keep #21, Bill Sharman. Hard to say with numbers from the 50's, but his look good enough to me.
I'd take down #22, Ed Macauley. Only six seasons with the team, and I've always hated the nickname "Easy Ed." Get back to me when you want to retire Tough-Guy Warrior Ed. (Yes, I'm kidding, but not about the verdict.)
#23, Frank Ramsey, is tough. First sixth man is historically significant. Still, the Celts have retired too many numbers from that era, and #23 is an awfully popular number since Jordan. I'd probably take it down.
#24, Sam Jones, has to stay up. Best guard in team history.
#25, KC Jones, should be taken down. I loved him as a coach, but he was a role player. It's like retiring James Posey's number. No, thanks.
#31 is Cedric Maxwell. Tough. Contributions to two champions matter. But only three years topping 15 PPG, only two years over 8 rebounds. Sorry, I think Cornbread goes.
#32 and #33 are obviously safe. If we can't retire McHale and Bird, the world's gone mad.
#34 hasn't been retired yet, but it obviously should be. Paul Pierce belongs in the rafters. I'm more on the fence about #5, since KG's legacy is in Minnesota as much as Boston. I probably wouldn't retire it here, but I'm fine with it happening. Judgment call.
#35 is Reggie Lewis. He only played big minutes in five seasons, but given the career he likely would have had if not for tragedy, I'll bow to the 'what-if' and say he should remain retired.
#00 is Robert Parish, and should stay retired. No-brainer.
So ideally, these are the numbers I think need to be in the rafters:
6, 10, 14, 15, 17, 18, 21, 24, 32, 33, 34, 35, 00. With an option on #5.
That's still quite a lot of retired numbers! And I think it would be a better reflection of the vital parts of Celtic history than the overkill we've got now.
But again, I don't favor actually doing this in the real world. You can't un-crack an egg.