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	$18.76	13.542%	$21.3005	$21.30	$21.30	$21.30
Purchase #2	$163.26	11.444%	$181.9435	$181.94	$181.95	$181.90
Purchase #3	$152.31	13.341%	$172.6297	$172.63	$172.65	$172.60
Purchase #4	$96.39	10.133%	$106.1572	$106.16	$106.15	$106.20
Purchase #5	$118.61	7.822%	$127.8877	$127.89	$127.90	$127.90
Purchase #6	$223.27	4.193%	$232.6317	$232.63	$232.65	$232.60
Purchase #7	$131.73	4.187%	$137.2455	$137.25	$137.25	$137.20
7 purchase totals =	$904.33		$979.7958	$979.80	$979.85	$979.70