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	$18.74	12.712%	$21.1222	$21.12	$21.10	$21.10
Purchase #2	$195.39	8.463%	$211.9259	$211.93	$211.95	$211.90
Purchase #3	$84.40	13.459%	$95.7594	$95.76	$95.75	$95.80
Purchase #4	$67.28	6.839%	$71.8813	$71.88	$71.90	$71.90
Purchase #5	$57.73	6.329%	$61.3837	$61.38	$61.40	$61.40
Purchase #6	$158.87	12.862%	$179.3039	$179.30	$179.30	$179.30
Purchase #7	$175.30	8.288%	$189.8289	$189.83	$189.85	$189.80
7 purchase totals =	$757.71		$831.2053	$831.20	$831.25	$831.20