I'd say it varies depending on your actual roster where you may not want to lose a promising guy even at a position of depth. At some level, it's like preserving an asset.
I think it's more of a range issue:
QB: usually 2, could see 3 here and there
RB: 3-5 is the range; I'd like 4
TE: 3-4
WR: 5-6
Ol: 9-11
DL: 8-10
LB: 6-8
DB: 9-11
ST: 3 (K, P, LS)