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	$18.71	13.156%	$21.1715	$21.17	$21.15	$21.20
Purchase #2	$155.41	7.151%	$166.5234	$166.52	$166.50	$166.50
Purchase #3	$166.42	6.303%	$176.9095	$176.91	$176.90	$176.90
Purchase #4	$151.85	10.345%	$167.5589	$167.56	$167.55	$167.60
Purchase #5	$51.59	13.229%	$58.4148	$58.41	$58.40	$58.40
Purchase #6	$79.72	11.760%	$89.0951	$89.10	$89.10	$89.10
Purchase #7	$89.84	11.619%	$100.2785	$100.28	$100.30	$100.30
7 purchase totals =	$713.54		$779.9517	$779.95	$779.90	$780.00