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	$71.77	11.457%	$79.9927	$79.99	$80.00	$80.00
Purchase #2	$226.78	12.785%	$255.7738	$255.77	$255.75	$255.80
Purchase #3	$234.98	12.233%	$263.7251	$263.73	$263.75	$263.70
Purchase #4	$93.75	7.228%	$100.5263	$100.53	$100.55	$100.50
Purchase #5	$234.38	5.156%	$246.4646	$246.46	$246.45	$246.50
Purchase #6	$60.31	3.543%	$62.4468	$62.45	$62.45	$62.40
Purchase #7	$79.77	4.690%	$83.5112	$83.51	$83.50	$83.50
7 purchase totals =	$1,001.74		$1,092.4405	$1,092.44	$1,092.45	$1,092.40