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	$73.44	9.249%	$80.2325	$80.23	$80.25	$80.20
Purchase #2	$246.72	7.612%	$265.5003	$265.50	$265.50	$265.50
Purchase #3	$72.78	7.276%	$78.0755	$78.08	$78.10	$78.10
Purchase #4	$116.44	9.429%	$127.4191	$127.42	$127.40	$127.40
Purchase #5	$144.82	3.680%	$150.1494	$150.15	$150.15	$150.10
Purchase #6	$7.54	9.314%	$8.2423	$8.24	$8.25	$8.20
Purchase #7	$115.50	13.573%	$131.1768	$131.18	$131.20	$131.20
7 purchase totals =	$777.24		$840.7959	$840.80	$840.85	$840.70