If they are giving it to a healer over an spriest then you are either really terrible or the healer is really undergeared and needs every bit of help he can get.
If my group is filled with warriors and dks or something then I will probably give it to a healer (if healer is rdruid or rsham), but if healer is hpal or dpriest i'll give it to the dk.
Basically I want someone who is critting often from dots or hots. There is a chart here on MMO-C in the lock forums that shows the numbers involved relating to DI targets.