I have been dps'ing as heals since level 30. I know the instances well and know the pulls or tanks that require me to purely heal, but if I can sneak a bane in, I'm helping.
It's been quite good practice.
Honestly, I don't hold back on any tank and the reason for this is that I know what he's going to encounter at 50. The ones "doing it wrong" may cuss at me under their breath, or in party chat, but when they reach 50, they'll owe me a thanks even if they don't realize it. If a tank is having agro problems every pull, i'll shut cleric stance off.
Also, most the tanks that lose hate to my dps agro would lose it to my healing agro too, because hitting overpower or flash more than once is really really hard.