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	$17.97	12.600%	$20.2342	$20.23	$20.25	$20.20
Purchase #2	$137.73	5.140%	$144.8093	$144.81	$144.80	$144.80
Purchase #3	$42.56	5.604%	$44.9451	$44.95	$44.95	$44.90
Purchase #4	$82.32	12.600%	$92.6923	$92.69	$92.70	$92.70
Purchase #5	$88.60	13.352%	$100.4299	$100.43	$100.45	$100.40
Purchase #6	$20.53	11.520%	$22.8951	$22.90	$22.90	$22.90
Purchase #7	$224.26	7.153%	$240.3013	$240.30	$240.30	$240.30
7 purchase totals =	$613.97		$666.3072	$666.31	$666.35	$666.20