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	$16.47	6.231%	$17.4962	$17.50	$17.50	$17.50
Purchase #2	$69.35	12.527%	$78.0375	$78.04	$78.05	$78.00
Purchase #3	$88.50	8.596%	$96.1075	$96.11	$96.10	$96.10
Purchase #4	$119.25	13.369%	$135.1925	$135.19	$135.20	$135.20
Purchase #5	$103.42	12.228%	$116.0662	$116.07	$116.05	$116.10
Purchase #6	$147.39	6.311%	$156.6918	$156.69	$156.70	$156.70
Purchase #7	$62.40	8.189%	$67.5099	$67.51	$67.50	$67.50
7 purchase totals =	$606.78		$667.1016	$667.11	$667.10	$667.10