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	$160.35	6.266%	$170.3975	$170.40	$170.40	$170.40
Purchase #2	$41.53	4.473%	$43.3876	$43.39	$43.40	$43.40
Purchase #3	$231.93	5.741%	$245.2451	$245.25	$245.25	$245.20
Purchase #4	$205.34	8.874%	$223.5619	$223.56	$223.55	$223.60
Purchase #5	$142.30	10.277%	$156.9242	$156.92	$156.90	$156.90
Purchase #6	$64.82	13.486%	$73.5616	$73.56	$73.55	$73.60
Purchase #7	$71.59	12.550%	$80.5745	$80.57	$80.55	$80.60
7 purchase totals =	$917.86		$993.6524	$993.65	$993.60	$993.70