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	$18.33	10.192%	$20.1982	$20.20	$20.20	$20.20
Purchase #2	$236.43	7.212%	$253.4813	$253.48	$253.50	$253.50
Purchase #3	$137.35	3.362%	$141.9677	$141.97	$141.95	$142.00
Purchase #4	$210.47	11.440%	$234.5478	$234.55	$234.55	$234.50
Purchase #5	$60.60	6.781%	$64.7093	$64.71	$64.70	$64.70
Purchase #6	$145.99	13.483%	$165.6738	$165.67	$165.65	$165.70
Purchase #7	$238.65	13.113%	$269.9442	$269.94	$269.95	$269.90
7 purchase totals =	$1,047.82		$1,150.5223	$1,150.52	$1,150.50	$1,150.50