Depends on the size and depth of the dent.....and the kind of bike it's on.....
I suspect that generally, the rarer/older the bike, the more people will be willing to ignore them..... but they still do affect the final market value on them.
BTW, there's "Rust Dog" bikes too. I suspect Aluminum and TI bikes can be "Crack Dogs", while CF bikes are "Asploding Dogs"!