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	$19.25	11.599%	$21.4828	$21.48	$21.50	$21.50
Purchase #2	$240.67	7.122%	$257.8105	$257.81	$257.80	$257.80
Purchase #3	$177.35	6.760%	$189.3389	$189.34	$189.35	$189.30
Purchase #4	$134.41	5.538%	$141.8536	$141.85	$141.85	$141.90
Purchase #5	$14.37	9.341%	$15.7123	$15.71	$15.70	$15.70
Purchase #6	$83.32	6.265%	$88.54	$88.54	$88.55	$88.50
Purchase #7	$76.28	13.428%	$86.5229	$86.52	$86.50	$86.50
7 purchase totals =	$745.65		$801.2610	$801.25	$801.25	$801.20