Healers are only really expected to DPS in super end-game content, such as Coils, but even then it's not mandatory. As a WHM if I do 100-0 DPS in a raid, my static isn't going to say boo to me about it 'cause I'm where most of their healing comes from and if I don't have the time to DPS, then I don't have the time to DPS. SCHs are more expected to DPS in these types of raids, but even then it's still not mandatory. I don't know many pugs for Coil who will kick a SCH if they're not doing about as much as a DPS (like about 300-400 or something), since they're a healer and that is their priority role.
In 4-man dungeon content, the only DPS a WHM is... not expected, but I suppose the term would be requested... to do is to throw out 1-3 Holy's on a trash pack. DPS and tanks really like that stun. But anything more than that is just icing on the cake and people aren't going to get mad at you if you don't do it.
DPS on a healer is totally optional and it is completely up to you if you would like to or not. :)