I think in more than 95% of the games you'll wanna use the coin token for one of the first 2 buys.
I'm going to go out on a limb and call it 99%. Consider the possibilities:
(1) You draw $2/$5. You can go $3/$5 or $2/$6. Opening with Gold or Goons is going to be pretty powerful most of the time. If there's a $5 that you'd want more than Gold (Wharf, Mountebank, etc.), well, you'd rather open Silver/$5 than $2/$5 most of the time.
The only major exceptions I can see are Hamlet/$5 and Chapel/$5, where the $5 is better than Gold.
(2) You draw $3/$4. You can go $3/$5 or $4/$4. If there's a power $4 on the board (Sea Hag, Tournament, etc.) then you want two of them on the first two turns, of course. If there's no power $4, then Silver/$5 is almost always going to be better than Silver/$4: the worst possible $5 on the board is Baker, which we've already established is at least as good as Peddler (someone else can be edge-case patrol for this).
The major exception I see is Double Ambassador, but we know that Amb/$4 Cantrip (Caravan, Tournament) is usually more powerful than Double Amb, and Baker is at least as good as Caravan.
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While I was writing this, SCSN pointed out that Alchemist and Familiar are another set of exceptions by themselves, which I suppose should have been obvious.