Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$239.72	13.122%	$271.1761	$271.18	$271.20	$271.20
Purchase #2	$55.85	7.104%	$59.8176	$59.82	$59.80	$59.80
Purchase #3	$50.48	6.728%	$53.8763	$53.88	$53.90	$53.90
Purchase #4	$110.47	6.313%	$117.444	$117.44	$117.45	$117.40
Purchase #5	$72.30	13.852%	$82.315	$82.32	$82.30	$82.30
Purchase #6	$49.30	8.532%	$53.5063	$53.51	$53.50	$53.50
Purchase #7	$35.27	13.590%	$40.0632	$40.06	$40.05	$40.10
7 purchase totals =	$613.39		$678.1985	$678.21	$678.20	$678.20