It might still work - especially if the dents are only slight and pretty small. Dry ice is also pretty cheap - can't hurt to give it a go - a lot cheaper than getting some bodywork done at a shop. As far as I know, also relatively safe for the paint - dry ice runs around ~ -110F - cold, but not too cold for modern automotive paint that has been fully cured. If in doubt, try it on a test spot and see if the paint cracks on you. I doubt it would - as I've played with liquid nitrogen before just fine - never had anything crack with the stuff (automotive, aerospace, electronics, etc.) - and that runs at ~ -320F.