Always, no. Usually, yes... except that you're less likely to have a mineable card with your mine if you have golds and coppers than if you have silvers and silvers.
But generally, you want to have more imbalance in your deck when you have lower money density, less imbalance when it's higher. And for classic BM strats, that generally means more imbalance is better.
Ah, now this is interesting.
So simulation-wise, we've established that Cache is a marginally better buy on $5 than silver if thats the only difference over a whole game.
Is there a simulation that will show us the best point when its best to stop buying caches and to go back to silvers?
Instinct tells me that by the time we reach that point, we're probably spending the $5 on Duchies...