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	$121.71	13.242%	$137.8268	$137.83	$137.85	$137.80
Purchase #2	$83.50	5.424%	$88.029	$88.03	$88.05	$88.00
Purchase #3	$234.69	9.321%	$256.5655	$256.57	$256.55	$256.60
Purchase #4	$249.31	13.640%	$283.3159	$283.32	$283.30	$283.30
Purchase #5	$93.49	12.611%	$105.28	$105.28	$105.30	$105.30
Purchase #6	$16.89	7.240%	$18.1128	$18.11	$18.10	$18.10
Purchase #7	$101.28	3.167%	$104.4875	$104.49	$104.50	$104.50
7 purchase totals =	$900.87		$993.6175	$993.63	$993.65	$993.60