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	$26.51	13.820%	$30.1737	$30.17	$30.15	$30.20
Purchase #2	$39.58	8.412%	$42.9095	$42.91	$42.90	$42.90
Purchase #3	$242.66	8.184%	$262.5193	$262.52	$262.50	$262.50
Purchase #4	$28.77	8.767%	$31.2923	$31.29	$31.30	$31.30
Purchase #5	$5.65	4.635%	$5.9119	$5.91	$5.90	$5.90
Purchase #6	$81.72	9.710%	$89.655	$89.66	$89.65	$89.70
Purchase #7	$168.90	4.300%	$176.1627	$176.16	$176.15	$176.20
7 purchase totals =	$593.79		$638.6244	$638.62	$638.55	$638.70