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	$24.75	12.251%	$27.7821	$27.78	$27.80	$27.80
Purchase #2	$123.48	6.411%	$131.3963	$131.40	$131.40	$131.40
Purchase #3	$244.50	10.601%	$270.4194	$270.42	$270.40	$270.40
Purchase #4	$67.80	4.544%	$70.8808	$70.88	$70.90	$70.90
Purchase #5	$130.30	11.790%	$145.6624	$145.66	$145.65	$145.70
Purchase #6	$64.62	3.465%	$66.8591	$66.86	$66.85	$66.90
Purchase #7	$95.31	7.289%	$102.2571	$102.26	$102.25	$102.30
7 purchase totals =	$750.76		$815.2572	$815.26	$815.25	$815.40