Timing is a major key, life tapping while the tank is pulling draws attention away from the task at hand. It also draws away from the healer's mana. My healer gets very frustrated when a Lock takes over his mana pool. To the point where he wanted to take a third rogue instead of a Warlock in a PUG. Life Tap is a cool ability in extended combat situations, but Warlocks can eat or drink at the end of a fight just like any other class.
Some healers don't mind, or even encourage Life Tapping. Lorina of Cenarian Circle mentioned that he asks Locks to Tap because it makes things go faster. Her comment to the OP concluded with "Be thankful you actually have healers that are concerned if you (as DPS) live or die, it's a rarity these days. And for good reason. "
I haven't healed a Warlock in ages, but as a tank it drives me crazy because I need my healer to maintain his mana pool and attention for my big bear butt. I'm also keenly aware of the fragility of a clothie in the red. I just have to remember our dungeon heuristic "If th healer dies, it's the tanks fault. If the tank dies it's the healer's fault. If the DPS dies, it's the DPS's fault." There are exceptions, but with Warlocks, this is spot on.
My best suggestion is to talk to the healer about his or her preference at the beginning of the instance. If you Life Tap inappropriately, don't be upset if you don't get healed.