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	$18.30	9.494%	$20.0374	$20.04	$20.05	$20.00
Purchase #2	$221.78	10.335%	$244.701	$244.70	$244.70	$244.70
Purchase #3	$152.79	4.605%	$159.826	$159.83	$159.85	$159.80
Purchase #4	$64.43	12.143%	$72.2537	$72.25	$72.25	$72.30
Purchase #5	$33.59	13.761%	$38.2123	$38.21	$38.20	$38.20
Purchase #6	$66.67	5.467%	$70.3148	$70.31	$70.30	$70.30
Purchase #7	$54.59	10.298%	$60.2117	$60.21	$60.20	$60.20
7 purchase totals =	$612.15		$665.5569	$665.55	$665.55	$665.50