This is an interesting topic, TP to the OP.
My vote goes to Sheed as the first big off the bench regardless with Big Baby Davis playing the bulk of the backup center minutes. This means if Perk leaves the floor, KG follows within a few minutes and vice versa. All four will play together some and overlap minutes.
I think one factor that hasnt been mentioned is Sheed's age. he is being talked about here by some as if he were 5 years younger. I anticipate great contributions from Sheed this year, but only for about 20-25 mpg. Anything more will be gravy IMO, but could also be dangerous to his durability come playoff time.
This is not to say he is washed up or anything, but I just dont see the ability to play big minutes anymore. While we are on the subject, Garnett may be looking at this sub 30 minute boat soon as well.
I am sure to be in the minority here, but if the minutes were something like: Perk 30, KG 30, Sheed 20, Davis 16, and Williams/Scalabrine for every possibly garbage minute that Big Baby doesnt get for example, I think we will be in great shape for the playoffs with an excellent record.