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	$18.82	13.789%	$21.4151	$21.42	$21.40	$21.40
Purchase #2	$250.34	12.104%	$280.6412	$280.64	$280.65	$280.60
Purchase #3	$134.88	10.574%	$149.1422	$149.14	$149.15	$149.10
Purchase #4	$210.65	8.503%	$228.5616	$228.56	$228.55	$228.60
Purchase #5	$155.38	10.355%	$171.4696	$171.47	$171.45	$171.50
Purchase #6	$92.73	9.482%	$101.5227	$101.52	$101.50	$101.50
Purchase #7	$153.83	11.723%	$171.8635	$171.86	$171.85	$171.90
7 purchase totals =	$1,016.63		$1,124.6159	$1,124.61	$1,124.55	$1,124.60