Vending machines would be problematic.
Most soda machines are charging $0.60 or $0.65 these days. Rounding to either $0.50 or $0.75 would be annoying.
That said, I think it'd be totally reasonable to eliminate all coins other than the nickel and quarter.
(If I really had my way, I'd also eliminate dollar bills (replacing with a dollar coin that doesn't suck, replace with some more valuable and small.... like dime-sized pure copper. Then eliminate all bills except the $5, $20, and $100).