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	$32.35	3.169%	$33.3752	$33.38	$33.40	$33.40
Purchase #2	$172.53	4.688%	$180.6182	$180.62	$180.60	$180.60
Purchase #3	$235.96	6.306%	$250.8396	$250.84	$250.85	$250.80
Purchase #4	$165.77	6.403%	$176.3843	$176.38	$176.40	$176.40
Purchase #5	$88.58	12.398%	$99.5621	$99.56	$99.55	$99.60
Purchase #6	$185.57	5.740%	$196.2217	$196.22	$196.20	$196.20
Purchase #7	$31.91	8.236%	$34.5381	$34.54	$34.55	$34.50
7 purchase totals =	$912.67		$971.5392	$971.54	$971.55	$971.50