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	$20.26	3.582%	$20.9857	$20.99	$21.00	$21.00
Purchase #2	$249.91	6.800%	$266.9039	$266.90	$266.90	$266.90
Purchase #3	$49.42	7.644%	$53.1977	$53.20	$53.20	$53.20
Purchase #4	$134.28	7.343%	$144.1402	$144.14	$144.15	$144.10
Purchase #5	$213.57	4.107%	$222.3413	$222.34	$222.35	$222.30
Purchase #6	$25.98	6.380%	$27.6375	$27.64	$27.65	$27.60
Purchase #7	$100.83	10.194%	$111.1086	$111.11	$111.10	$111.10
7 purchase totals =	$794.25		$846.3149	$846.32	$846.35	$846.20