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	$6.69	12.722%	$7.5411	$7.54	$7.55	$7.50
Purchase #2	$207.68	9.754%	$227.9371	$227.94	$227.95	$227.90
Purchase #3	$83.38	9.577%	$91.3653	$91.37	$91.35	$91.40
Purchase #4	$89.44	11.103%	$99.3705	$99.37	$99.35	$99.40
Purchase #5	$9.49	12.799%	$10.7046	$10.70	$10.70	$10.70
Purchase #6	$81.76	12.178%	$91.7167	$91.72	$91.70	$91.70
Purchase #7	$236.79	11.314%	$263.5804	$263.58	$263.60	$263.60
7 purchase totals =	$715.23		$792.2157	$792.22	$792.20	$792.20