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	$34.76	12.524%	$39.1133	$39.11	$39.10	$39.10
Purchase #2	$126.61	13.381%	$143.5517	$143.55	$143.55	$143.60
Purchase #3	$243.41	13.427%	$276.0927	$276.09	$276.10	$276.10
Purchase #4	$218.97	4.497%	$228.8171	$228.82	$228.80	$228.80
Purchase #5	$19.51	7.361%	$20.9461	$20.95	$20.95	$20.90
Purchase #6	$186.71	11.634%	$208.4318	$208.43	$208.45	$208.40
Purchase #7	$176.72	6.728%	$188.6097	$188.61	$188.60	$188.60
7 purchase totals =	$1,006.69		$1,105.5624	$1,105.56	$1,105.55	$1,105.50