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	$28.62	8.166%	$30.9571	$30.96	$30.95	$31.00
Purchase #2	$164.81	11.277%	$183.3956	$183.40	$183.40	$183.40
Purchase #3	$126.43	10.493%	$139.6963	$139.70	$139.70	$139.70
Purchase #4	$142.73	11.798%	$159.5693	$159.57	$159.55	$159.60
Purchase #5	$47.31	7.419%	$50.8199	$50.82	$50.80	$50.80
Purchase #6	$127.25	10.672%	$140.8301	$140.83	$140.85	$140.80
Purchase #7	$76.64	8.142%	$82.88	$82.88	$82.90	$82.90
7 purchase totals =	$713.79		$788.1483	$788.16	$788.15	$788.20