The effect is tied to the damage, so no, you needn't flip the coin if you are undamaged - including through the use of Iron Skin.
I have a follow up question to this. When you're using this potion, I assume you cannot use any of the redirect action cards either as they would have no affect (My Hero, Human Shield).
Can you not use Counter Strike against the creature either, since you're not being damaged?
It seems like a very slight nuance, since there is the Poke with a Stick card that also has a value of zero but still is a valid attack. I noticed the Iron Skin text reads that damage is reduced to zero (instead of saying attacks have no affect). Not sure if that specific wording is intentional.