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	$39.42	6.777%	$42.0915	$42.09	$42.10	$42.10
Purchase #2	$21.72	11.871%	$24.2984	$24.30	$24.30	$24.30
Purchase #3	$249.74	5.730%	$264.0501	$264.05	$264.05	$264.10
Purchase #4	$63.80	10.208%	$70.3127	$70.31	$70.30	$70.30
Purchase #5	$34.44	7.403%	$36.9896	$36.99	$37.00	$37.00
Purchase #6	$44.39	3.129%	$45.779	$45.78	$45.80	$45.80
Purchase #7	$35.30	5.614%	$37.2817	$37.28	$37.30	$37.30
7 purchase totals =	$488.81		$520.8030	$520.80	$520.85	$520.90