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	$84.72	5.316%	$89.2237	$89.22	$89.20	$89.20
Purchase #2	$78.51	3.459%	$81.2257	$81.23	$81.25	$81.20
Purchase #3	$67.37	7.668%	$72.5359	$72.54	$72.55	$72.50
Purchase #4	$147.30	9.229%	$160.8943	$160.89	$160.90	$160.90
Purchase #5	$25.37	13.556%	$28.8092	$28.81	$28.80	$28.80
Purchase #6	$60.72	13.861%	$69.1364	$69.14	$69.15	$69.10
Purchase #7	$69.43	13.350%	$78.6989	$78.70	$78.70	$78.70
7 purchase totals =	$533.42		$580.5241	$580.53	$580.55	$580.40