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	$19.67	9.171%	$21.4739	$21.47	$21.45	$21.50
Purchase #2	$194.54	3.237%	$200.8373	$200.84	$200.85	$200.80
Purchase #3	$236.34	6.145%	$250.8631	$250.86	$250.85	$250.90
Purchase #4	$57.58	4.758%	$60.3197	$60.32	$60.30	$60.30
Purchase #5	$66.61	6.535%	$70.963	$70.96	$70.95	$71.00
Purchase #6	$24.85	3.191%	$25.643	$25.64	$25.65	$25.60
Purchase #7	$178.37	7.332%	$191.4481	$191.45	$191.45	$191.40
7 purchase totals =	$777.96		$821.5481	$821.54	$821.50	$821.50