Off spec or Side spec?

How do you deal with people who want to take gear for their second (or more) set for their main spec? If you use DKP, how do you organise the bidding?

For example: tanks who want a threat set, a mitigation set, AND an avoidance set? Is that considered greedy if there are other people in the raid who need stuff because it’s just an all around upgrade for whatever they have? Or people who might want healing gear for an offspec vs healers who want a few pieces with extra crit that they don’t intend to use very often?