My rule of thumb is: How much is my job worth?

Cheating on your expense report by $5 or $10 doesn't seem like much by itself, but is it worth $5 or $10 to you to keep your job? Falsifying your expense report is one of those black-and-white things that can cause your boss to *have* to fire you if HR or your auditors catch it.

That said, it's not a black-and-white world, and we all have to live by what I think of as our internal ledger books. For instance, I make the occasional personal long-distance call from my office phone - but I don't charge my company mileage for my occasional in-town travel. I figure it about evens out.

Changing HTML code feels like crossing a line to me. Have you thought about having the conversation with your HR person ahead of time? If the two of you agree that 1st is ok if it's less than full-fare coach or coach is not available, then all you have to do is scribble a short note of explanation on the receipt each time and you're covered.