I'm thinking Taxman mirror games will lead to some interesting tactics. Should I Tax Copper and be more likely to hit you, or Silver and do more harm? If I topdeck a Silver, you'll know I have that in my hand, which could be bad if you have one to Tax. Then again, if I turn Silver into Gold at every opportunity, I'll have less flexibility to Tax Silver myself when similar opportunities arise.
Oh wow, I hadn't noticed that mirror synergy before. Your opponent will know a treasure in your hand the turn after you use Taxman. That will really change up how it plays. Previously, I figured I'd nearly always go with Copper.
I think are a couple other factors in favor of going Silver->Gold:
Your opponent is rarely going to want to Tax Gold into Gold (although occasionally it might be good to shuffle a Gold over to the next hand). Accordingly, your Gold is relatively safe from getting attacked, except of course in Platinum games. But if you give yourself a deck full of Silver, they'll surely stop attacking your Coppers and start Taxing Silver->Gold, and then you might be in for a rough time.
Also, Taxman might actually go well with other mild trashers, like say Lookout. This might seem counterintuitive but Taxman is an even slower card than Mine: it's not as though you're going to upgrade all of your starting Coppers into Golds with one Taxman, are you? Why not drop a few? It'll help you avoid your opponent's Taxmen, and if you really need, you can always pick up a couple Silvers when you start running out of Taxman targets.