It is damn near a universal truth that it is NEVER a good idea to Life Tap to full mana while raiding. When you Life Tap to full mana, not only are you pulling your health down further than it would be if you were to Life Tap to 70-90 percent mana, you are also wasting the spirit which would otherwise have continued to replenish your mana anyway. As a result your healer will need to use their resources to heal the resulting discrepancy in health which would not have been a factor had you not used Life Tap to completely replenish your mana. This is a waste of the groups resources.
Instead, use Life Tap until you have either 80% mana or 30% health, whichever occurs first. But when using Life Tap, keep an eye on the tanks health and the healers mana. Because if the tank is low on health, he will always take priority. If the healer is low on mana, the tank will always take priority.
Also consider that once your spirit has replenished your mana to 100% and you still have less than full health, you have essentially wasted your mana regen AND the difference in health.
I'm not playing Warlock but I used to and I see far too many warlocks do this, much to the detriment of their own effectiveness.
SUMMARY:
Don't waste your mana regen by Life Tapping to full mana when it's almost never necessary. Life Tap to 80% mana instead, then again when you're low on mana if necessary.