Timing your Thornmail purchase correctly, not just "who should I get it against"
Thornmail is more than just getting it against physical-heavy teams or as Rammus; I always run into a conundrum when wanting to get it. If you get it early, you have no HP to back it up, so any magic damage and abilities are going to wreck you. But if you get Thornmail late, your opponents are likely already going to have enough lifesteal and/or MR to make its effect borderline worthless.
This is where the Thornmail trap reputation comes from. There seems to be no optimal time to get it without screwing yourself in some way.
So which timing is going to bite you in the ass LESS?