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	$39.53	3.445%	$40.8918	$40.89	$40.90	$40.90
Purchase #2	$91.78	12.594%	$103.3388	$103.34	$103.35	$103.30
Purchase #3	$227.49	12.373%	$255.6373	$255.64	$255.65	$255.60
Purchase #4	$119.41	12.471%	$134.3016	$134.30	$134.30	$134.30
Purchase #5	$232.32	11.139%	$258.1981	$258.20	$258.20	$258.20
Purchase #6	$227.77	12.266%	$255.7083	$255.71	$255.70	$255.70
Purchase #7	$186.74	3.102%	$192.5327	$192.53	$192.55	$192.50
7 purchase totals =	$1,125.04		$1,240.6086	$1,240.61	$1,240.65	$1,240.50