If a raid goes south, and mnemonic potions are consumed, I will always pony up and replace potions. If I'm leading a raid, well... that's my responsibility, plain and simple.
But, even if I'm not leading the raid, I do it. Why? Because I want other healers to remember my assorted characters as being generous so that they will will think "hey, that guy reimbursed me potions - I want to group with that guy!"
Plus, it's simply the right thing to do.