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	$12.79	5.723%	$13.522	$13.52	$13.50	$13.50
Purchase #2	$207.31	5.304%	$218.3057	$218.31	$218.30	$218.30
Purchase #3	$231.82	6.501%	$246.8906	$246.89	$246.90	$246.90
Purchase #4	$197.35	11.217%	$219.4867	$219.49	$219.50	$219.50
Purchase #5	$109.61	9.406%	$119.9199	$119.92	$119.90	$119.90
Purchase #6	$67.83	13.581%	$77.042	$77.04	$77.05	$77.00
Purchase #7	$24.81	10.511%	$27.4178	$27.42	$27.40	$27.40
7 purchase totals =	$851.52		$922.5847	$922.59	$922.55	$922.50