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	$39.51	7.364%	$42.4195	$42.42	$42.40	$42.40
Purchase #2	$130.36	12.670%	$146.8766	$146.88	$146.90	$146.90
Purchase #3	$229.53	11.245%	$255.3406	$255.34	$255.35	$255.30
Purchase #4	$35.35	3.494%	$36.5851	$36.59	$36.60	$36.60
Purchase #5	$123.91	4.689%	$129.7201	$129.72	$129.70	$129.70
Purchase #6	$194.46	9.538%	$213.0076	$213.01	$213.00	$213.00
Purchase #7	$109.60	10.309%	$120.8987	$120.90	$120.90	$120.90
7 purchase totals =	$862.72		$944.8482	$944.86	$944.85	$944.80