A beat up coin is going to fetch less of a premium over spot but is still always worth the melt value provided it is still intact (no holes or missing metal). Why? People like pretty things better than ugly ones. If you had a choice between two identically equipped vehicles, one that was shiny blue and the other one was faded puke green, which would you choose? which would sell for more in an auction?
With junk silver, older coins are sometimes thinner than newer ones, which can affect the value. The last two rolls I bought were sent bagged by decade and I put them in tubes oldest to newest. The second (newer) roll of 40 was more than 1/4 inch taller than the first roll. So by melt value 1964 quarters might be worth more than ones from the 30s and 40s.
hth,
eb