I guess I take a proactive approach to Lifetapping 'locks since I play a Priest as a main and alt a Warlock. When I'm on my Priest, I tell the Warlock they can LT as needed and I'll heal them up. I recommend they use Rank 1 Life Tap if they're just using it as part of their rotation. I'll usually just keep Renew running on the warlock at all times, and that's enough to keep their health up. If they LT more than my Renew can handle, that's on them. With pets, I have pet frames on my VuhDo grid, and will heal and buff them as needed - I understand they're an important part of DPS and hey, more DPS helps make my job easier too.

When I play my 'lock, I do get annoyed if a healer doesn't bother tossing me a HoT or a quick heal now and then. I don't LT excessively with her, and I use Rank 1 LT in my DPS rotation. I usually keep my 'lock at about 75% mana. If it's after a hard fight and I get below that, then yeah I eat/drink - it's not a big deal. But having played both sides, and knowing how many times a Warlock should need to LT, as well as knowing how much a simple HoT can heal for, there's no reason that a healer shouldn't be able to toss a Warlock one to top them off, nor is there any reason for a Warlock to LT down to dangerous levels.