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	$19.99	7.152%	$21.4197	$21.42	$21.40	$21.40
Purchase #2	$236.39	12.410%	$265.726	$265.73	$265.75	$265.70
Purchase #3	$27.46	7.135%	$29.4193	$29.42	$29.40	$29.40
Purchase #4	$238.45	6.732%	$254.5025	$254.50	$254.50	$254.50
Purchase #5	$160.74	11.619%	$179.4164	$179.42	$179.40	$179.40
Purchase #6	$56.45	4.839%	$59.1816	$59.18	$59.20	$59.20
Purchase #7	$8.94	10.675%	$9.8943	$9.89	$9.90	$9.90
7 purchase totals =	$748.42		$819.5598	$819.56	$819.55	$819.50