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	$6.42	3.182%	$6.6243	$6.62	$6.60	$6.60
Purchase #2	$89.76	3.474%	$92.8783	$92.88	$92.90	$92.90
Purchase #3	$11.76	9.791%	$12.9114	$12.91	$12.90	$12.90
Purchase #4	$246.87	4.625%	$258.2877	$258.29	$258.30	$258.30
Purchase #5	$188.88	9.772%	$207.3374	$207.34	$207.35	$207.30
Purchase #6	$72.57	5.497%	$76.5592	$76.56	$76.55	$76.60
Purchase #7	$11.54	9.585%	$12.6461	$12.65	$12.65	$12.60
7 purchase totals =	$627.80		$667.2444	$667.25	$667.25	$667.20