I doubt Americans will have long to get used to rounding to the nearest nickel. They already cost 14¢ to produce and it won't be getting cheaper.
Discussion
But what will this do to collector price of pennies?
Haha. They sell giant bricks of million dollar bills for pennies after hyperinflation events. Even as sentimental collectors items I doubt they'd be worth the 4¢ they take to mint.
1960s real silver quarters still sell for the melt value of their silver weight in the collector community. No premium to be found except in the most pristine of specimens.