IMO, if you queue as heals but don't heal you are placing a burden on the tank and other healers. You are taking the fast queue, but not doing the extra work. If the raid wipes, they are well within reason to kick you. Same with tanking, don't queue as a tank and expect to DPS. Play the role you queued for, or queue for the role you want to play.
My exceptions to this are:
When everyone overgears the content, and you communicated your intent to the raid first (however, wipes are still on you, you picked wrong),
When some of the raid is missing and DPS step up to tank/heal until the queue is filled again, and you announced your intent
When one tank completely outgears the content, and the other tank negotiated changing to DPS
TL;DR - Don't be selfish, play what you queue. If you do change spec mid-raid, tell the raid.