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	$38.55	9.757%	$42.3113	$42.31	$42.30	$42.30
Purchase #2	$144.38	8.454%	$156.5859	$156.59	$156.60	$156.60
Purchase #3	$32.79	10.443%	$36.2143	$36.21	$36.20	$36.20
Purchase #4	$113.46	13.258%	$128.5025	$128.50	$128.50	$128.50
Purchase #5	$34.66	13.713%	$39.4129	$39.41	$39.40	$39.40
Purchase #6	$16.38	6.513%	$17.4468	$17.45	$17.45	$17.40
Purchase #7	$90.28	11.203%	$100.3941	$100.39	$100.40	$100.40
7 purchase totals =	$470.50		$520.8678	$520.86	$520.85	$520.80