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	$81.44	8.746%	$88.5627	$88.56	$88.55	$88.60
Purchase #2	$38.65	4.641%	$40.4437	$40.44	$40.45	$40.40
Purchase #3	$249.83	11.769%	$279.2325	$279.23	$279.25	$279.20
Purchase #4	$36.59	5.399%	$38.5655	$38.57	$38.55	$38.60
Purchase #5	$84.30	5.524%	$88.9567	$88.96	$88.95	$89.00
Purchase #6	$234.54	13.806%	$266.9206	$266.92	$266.90	$266.90
Purchase #7	$173.88	7.479%	$186.8845	$186.88	$186.90	$186.90
7 purchase totals =	$899.23		$989.5662	$989.56	$989.55	$989.60