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	$239.40	6.218%	$254.2859	$254.29	$254.30	$254.30
Purchase #2	$103.72	9.469%	$113.5412	$113.54	$113.55	$113.50
Purchase #3	$19.41	8.664%	$21.0917	$21.09	$21.10	$21.10
Purchase #4	$55.92	7.120%	$59.9015	$59.90	$59.90	$59.90
Purchase #5	$16.32	13.655%	$18.5485	$18.55	$18.55	$18.50
Purchase #6	$54.35	11.164%	$60.4176	$60.42	$60.40	$60.40
Purchase #7	$19.31	12.753%	$21.7726	$21.77	$21.75	$21.80
7 purchase totals =	$508.43		$549.5590	$549.56	$549.55	$549.50