> If we eliminate the nickel and the penny at the same time, you can bet that prices will be rounded up to the nearest dime.

Yes, this. I probably buy 40-60 items at the supermarket in a standard trip. I make that grocery trip once a week.
So if the supermarket adjusts all prices to be a multiple of 10c, they'll probably be rounding up by 5c on each item - which would be $130/year in extra grocery costs alone.