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	$84.76	6.656%	$90.4016	$90.40	$90.40	$90.40
Purchase #2	$75.96	12.380%	$85.3638	$85.36	$85.35	$85.40
Purchase #3	$154.29	8.581%	$167.5296	$167.53	$167.55	$167.50
Purchase #4	$200.67	4.242%	$209.1824	$209.18	$209.20	$209.20
Purchase #5	$82.89	7.195%	$88.8539	$88.85	$88.85	$88.90
Purchase #6	$168.96	13.320%	$191.4655	$191.47	$191.45	$191.50
Purchase #7	$6.73	5.795%	$7.12	$7.12	$7.10	$7.10
7 purchase totals =	$774.26		$839.9168	$839.91	$839.90	$840.00