Thanks all for your responses.

Just so you know where I was coming from, if you put a new m/c chain on worn sprockets, you'll have drastically accelerated chain wear because of the worn teeth. It could shift just fine at first (yes, shifting is where you notice it first, even though the chain doesn't get displaced - because of inconsistent tension on the countershaft sprocket and therefore on the transmission gears). But you'd only get half the expected life out of the chain, or less.

On a b/c, obviously the forces are much less, but the materials are also much smaller and lighter, so I wasn't sure.

And come to think of it... a set of m/c sprockets costs less than a chain of comparable quality. So you don't want to wear out the chain prematurely if you don't have to. On a b/c, obviously chainrings and cassettes cost way more than chains. That alone answers my question, I suppose. See what happens when you think about something?