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	$79.43	13.348%	$90.0323	$90.03	$90.05	$90.00
Purchase #2	$36.75	7.468%	$39.4945	$39.49	$39.50	$39.50
Purchase #3	$138.25	4.425%	$144.3676	$144.37	$144.35	$144.40
Purchase #4	$84.50	6.553%	$90.0373	$90.04	$90.05	$90.00
Purchase #5	$45.59	10.840%	$50.532	$50.53	$50.55	$50.50
Purchase #6	$7.74	5.816%	$8.1902	$8.19	$8.20	$8.20
Purchase #7	$75.58	9.669%	$82.8878	$82.89	$82.90	$82.90
7 purchase totals =	$467.84		$505.5417	$505.54	$505.60	$505.50