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).
The tax is applies after the total is added up so there is no other choice When I was a kid a value meal at McDonalds was $3.18 after tax, but two of them was $6.37 - that extra penny over all the meals McDonalds sells in a state adds up to a lot of money and they were not going to let that go.
