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	$18.61	13.161%	$21.0593	$21.06	$21.05	$21.10
Purchase #2	$159.46	6.709%	$170.1582	$170.16	$170.15	$170.20
Purchase #3	$40.30	12.393%	$45.2944	$45.29	$45.30	$45.30
Purchase #4	$158.79	12.136%	$178.0608	$178.06	$178.05	$178.10
Purchase #5	$89.37	7.684%	$96.2372	$96.24	$96.25	$96.20
Purchase #6	$38.67	3.801%	$40.1398	$40.14	$40.15	$40.10
Purchase #7	$9.31	10.684%	$10.3047	$10.30	$10.30	$10.30
7 purchase totals =	$514.51		$561.2544	$561.25	$561.25	$561.30