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	$72.76	10.324%	$80.2717	$80.27	$80.25	$80.30
Purchase #2	$80.54	10.801%	$89.2391	$89.24	$89.25	$89.20
Purchase #3	$128.62	8.219%	$139.1913	$139.19	$139.20	$139.20
Purchase #4	$50.82	8.297%	$55.0365	$55.04	$55.05	$55.00
Purchase #5	$73.55	11.261%	$81.8325	$81.83	$81.85	$81.80
Purchase #6	$84.47	6.233%	$89.735	$89.74	$89.75	$89.70
Purchase #7	$110.68	13.414%	$125.5266	$125.53	$125.55	$125.50
7 purchase totals =	$601.44		$660.8327	$660.84	$660.90	$660.70