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	$155.43	6.410%	$165.3931	$165.39	$165.40	$165.40
Purchase #2	$81.41	12.259%	$91.3901	$91.39	$91.40	$91.40
Purchase #3	$202.36	4.673%	$211.8163	$211.82	$211.80	$211.80
Purchase #4	$166.62	11.281%	$185.4164	$185.42	$185.40	$185.40
Purchase #5	$147.79	7.511%	$158.8905	$158.89	$158.90	$158.90
Purchase #6	$93.71	10.853%	$103.8803	$103.88	$103.90	$103.90
Purchase #7	$71.46	13.377%	$81.0192	$81.02	$81.00	$81.00
7 purchase totals =	$918.78		$997.8059	$997.81	$997.80	$997.80