Why not just pay the fine and learn your lesson for next time? If you think you're going to be travelling again, is it really worth the risk and the worrying every time you do so? I would guess the fine is not that much, probably less then Euro 50 or so? If you can afford to travel to Italy, you can probably afford to pay the fine.
I totally get that it feels like you were screwed over and that you are giving in by paying, so it's up to you have much it's worth to you to stick to your guns. Life isn't always fair.
Update: Based on your comments, I am not sure that you understand how those bus passes work. It's not just valid 90 minutes from when you purchase, it's valid 90 minutes from when you validate it (start your journey). So by not validating, you could theoretically use the pass again afterwards. This is very typical in much of Europe, and getting fined for not validating is also normal. You can't expect the bus driver to keep track of everyone who gets on the bus and when.