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	$17.31	9.243%	$18.91	$18.91	$18.90	$18.90
Purchase #2	$217.40	10.689%	$240.6379	$240.64	$240.65	$240.60
Purchase #3	$94.77	6.408%	$100.8429	$100.84	$100.85	$100.80
Purchase #4	$29.74	8.688%	$32.3238	$32.32	$32.30	$32.30
Purchase #5	$135.27	11.107%	$150.2944	$150.29	$150.30	$150.30
Purchase #6	$33.70	13.598%	$38.2825	$38.28	$38.30	$38.30
Purchase #7	$26.29	13.160%	$29.7498	$29.75	$29.75	$29.70
7 purchase totals =	$554.48		$611.0413	$611.03	$611.05	$610.90