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	$80.39	12.545%	$90.4749	$90.47	$90.45	$90.50
Purchase #2	$159.27	6.601%	$169.7834	$169.78	$169.80	$169.80
Purchase #3	$135.41	13.137%	$153.1988	$153.20	$153.20	$153.20
Purchase #4	$80.92	9.666%	$88.7417	$88.74	$88.75	$88.70
Purchase #5	$30.30	11.664%	$33.8342	$33.83	$33.85	$33.80
Purchase #6	$46.83	12.681%	$52.7685	$52.77	$52.75	$52.80
Purchase #7	$67.28	5.723%	$71.1304	$71.13	$71.15	$71.10
7 purchase totals =	$600.40		$659.9319	$659.92	$659.95	$659.90