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	$51.46	7.383%	$55.2593	$55.26	$55.25	$55.30
Purchase #2	$134.86	5.367%	$142.0979	$142.10	$142.10	$142.10
Purchase #3	$199.95	3.288%	$206.5244	$206.52	$206.50	$206.50
Purchase #4	$141.52	9.303%	$154.6856	$154.69	$154.70	$154.70
Purchase #5	$121.94	12.805%	$137.5544	$137.55	$137.55	$137.60
Purchase #6	$54.55	9.719%	$59.8517	$59.85	$59.85	$59.90
Purchase #7	$195.89	13.275%	$221.8944	$221.89	$221.90	$221.90
7 purchase totals =	$900.17		$977.8677	$977.86	$977.85	$978.00