The last-ever penny will be minted today in Philadelphia

We eliminated pennies in Canada in 2012 and the transition was a non-issue. The vast majority of retailers would round cash transactions to the nearest $0.05, but a few would round down to the nearest $0.05 in favor of the customer. https://en.wikipedia.org/wiki/Withdrawal_of_low-denomination...

Canadian cash is better than American cash in several ways: No penny, durable polymer banknotes (instead of dirty wrinkly cotton paper), colorful banknotes (instead of all green) that are easy to distinguish, $1 and $2 coins in wide circulation (instead of worn-out $1 bills).

I honestly don't know why we don't get rid of nickels and dimes as well. What can you still buy that costs less than $0.25?

Yes, the quarter is pretty much the smallest useful unit of US currency and even that usefulness is shrinking pretty quickly.

If we would adopt a policy of including local sales tax in advertised prices, skipping to whole dollars would be pretty painless.

The main reason to keep at least quarters is all of the various coin-op machines that are still in service.

The US has too many tax permutations for this to be practicable. Companies would have to make prices a bit higher to accommodate unexpected sales tax increases in some or other jurisdiction.

There's a small industry that specializes in knowing what the sales tax for a particular transaction should be at the moment it goes through.