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	$18.88	11.311%	$21.0155	$21.02	$21.00	$21.00
Purchase #2	$77.62	6.567%	$82.7173	$82.72	$82.70	$82.70
Purchase #3	$223.90	10.419%	$247.2281	$247.23	$247.25	$247.20
Purchase #4	$40.93	11.155%	$45.4957	$45.50	$45.50	$45.50
Purchase #5	$161.49	3.307%	$166.8305	$166.83	$166.85	$166.80
Purchase #6	$123.72	7.865%	$133.4506	$133.45	$133.45	$133.50
Purchase #7	$136.44	11.128%	$151.623	$151.62	$151.60	$151.60
7 purchase totals =	$782.98		$848.3607	$848.37	$848.35	$848.30