There's apparently a technical problem with accepting pennies. The "silver" coins have an electronic signature that can be detected easily, so slugs can be rejected. Copper pennies are harder to validate. It's possible that the current copper-clad zinc pennies can be handled, but any older pennies would be rejected.

Rounding to the nearest 5c today causes the same rounding error as rounding to 1c in 1971. Prior to that, the rounding error was greater than rounding to a nickel would be today.