Funny this was brought up.
I did just this very thing today at a park. This park must have never been hunted. I couldn't take 1 step without finding 3-5 coins. I found so many pennys I decided to pass on em. The dates on all the coins were newer than 1975. It was raining light so i didn't have much time as i though, So i just went to the silver. I found more quarters than dimes and nickles. Pennys were on the surface many times I just gave up and went for the good stuff. Ended up with $5 bucks in change in less than two hours and I still didn't get em all.
So if its all newer change and your time is constrained, i think then as i did today, would discriminate pennies. I wouldn't do it if i had all the time in the world.