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	$39.99	5.594%	$42.227	$42.23	$42.25	$42.20
Purchase #2	$83.79	7.625%	$90.179	$90.18	$90.20	$90.20
Purchase #3	$60.87	6.551%	$64.8576	$64.86	$64.85	$64.90
Purchase #4	$59.37	7.187%	$63.6369	$63.64	$63.65	$63.60
Purchase #5	$250.33	13.737%	$284.7178	$284.72	$284.70	$284.70
Purchase #6	$50.83	12.731%	$57.3012	$57.30	$57.30	$57.30
Purchase #7	$69.54	10.620%	$76.9251	$76.93	$76.95	$76.90
7 purchase totals =	$614.72		$679.8446	$679.86	$679.90	$679.80