If it means shops stop charging $4.99 and start charging $5.00, I will be ecstatic.

Problem is it isn't just the $5.99 rounds to $6.00 it is tax. If the end cost is $6.36 will the state be happy with that one penny less? For any state 1 penny per transaction is millions of dollars per year! (note that I had to change your price from 4.99 to 5.99 - 5.00 times any tax rate is an even multiple of 5 and so cannot make the point).