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	$5.70	10.244%	$6.2839	$6.28	$6.30	$6.30
Purchase #2	$18.57	9.784%	$20.3869	$20.39	$20.40	$20.40
Purchase #3	$94.28	4.669%	$98.6819	$98.68	$98.70	$98.70
Purchase #4	$64.32	10.673%	$71.1849	$71.18	$71.20	$71.20
Purchase #5	$214.48	8.385%	$232.4641	$232.46	$232.45	$232.50
Purchase #6	$73.77	12.274%	$82.8245	$82.82	$82.80	$82.80
Purchase #7	$116.65	9.565%	$127.8076	$127.81	$127.80	$127.80
7 purchase totals =	$587.77		$639.6338	$639.62	$639.65	$639.70