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	$17.73	4.592%	$18.5442	$18.54	$18.55	$18.50
Purchase #2	$87.57	11.282%	$97.4496	$97.45	$97.45	$97.40
Purchase #3	$105.70	7.146%	$113.2533	$113.25	$113.25	$113.30
Purchase #4	$78.43	4.334%	$81.8292	$81.83	$81.85	$81.80
Purchase #5	$138.73	11.808%	$155.1112	$155.11	$155.10	$155.10
Purchase #6	$37.97	7.478%	$40.8094	$40.81	$40.80	$40.80
Purchase #7	$30.77	9.754%	$33.7713	$33.77	$33.75	$33.80
7 purchase totals =	$496.90		$540.7682	$540.76	$540.75	$540.70