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	$126.91	9.511%	$138.9804	$138.98	$139.00	$139.00
Purchase #2	$55.90	6.118%	$59.32	$59.32	$59.30	$59.30
Purchase #3	$40.32	4.781%	$42.2477	$42.25	$42.25	$42.20
Purchase #4	$243.97	10.371%	$269.2721	$269.27	$269.25	$269.30
Purchase #5	$36.64	5.288%	$38.5775	$38.58	$38.60	$38.60
Purchase #6	$16.57	8.342%	$17.9523	$17.95	$17.95	$18.00
Purchase #7	$209.47	13.728%	$238.226	$238.23	$238.25	$238.20
7 purchase totals =	$729.78		$804.5760	$804.58	$804.60	$804.60