My main is a priest specced heals/heals and my alt is a druid specced heals/feral-dps; and I only really use the first spec on my druid. So this is the perspective of a dedicated (obsessed perhaps) heal-bitch: it's not a massive problem, but I'd rather you didn't.
If the healers are still alive (and are competent) there should be almost no reason for DPS to heal in a raid. For most healers the biggest issue that we face is mana, so we're generally hoping to get things done faster. Having DPS stop doing damage and start healing just slows everything down, and 90% of the time the heal they cast doesn't land because someone else got there first. And if it does land, it's generally because it sniped a heal that one of the dedicated healers was casting, which just means that you're wasting your mana; that is, "raid mana" that could have been used to do damage just went to healing, while healing mana didn't go to damage.
What you're doing is the sign of a good player in my book; you're trying to maximise your contribution to the raid. The trouble is that, unless you are watching the damage all the time, you can't tell what is a continuous spike and what's just a one-hit spike. How those get handled by the main healers is going to depend on what's happening at the time. You might see someone in "need" of healing and start trying to save them, but I'm already aware that they're low on health and am deliberately leaving them a few moments while I help someone in immediate need.
It's a preference thing I guess, as I see that other healers here disagree with me. That's just my 2 copper.