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	$75.31	6.242%	$80.0109	$80.01	$80.00	$80.00
Purchase #2	$95.92	9.617%	$105.1446	$105.14	$105.15	$105.10
Purchase #3	$74.51	7.720%	$80.2622	$80.26	$80.25	$80.30
Purchase #4	$32.49	4.209%	$33.8575	$33.86	$33.85	$33.90
Purchase #5	$187.78	7.575%	$202.0043	$202.00	$202.00	$202.00
Purchase #6	$83.80	12.300%	$94.1074	$94.11	$94.10	$94.10
Purchase #7	$236.35	3.196%	$243.9037	$243.90	$243.90	$243.90
7 purchase totals =	$786.16		$839.2906	$839.28	$839.25	$839.30