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	$240.52	12.390%	$270.3204	$270.32	$270.30	$270.30
Purchase #2	$215.35	11.555%	$240.2337	$240.23	$240.25	$240.20
Purchase #3	$113.64	12.364%	$127.6904	$127.69	$127.70	$127.70
Purchase #4	$201.93	3.315%	$208.624	$208.62	$208.60	$208.60
Purchase #5	$211.25	13.857%	$240.5229	$240.52	$240.50	$240.50
Purchase #6	$57.61	13.282%	$65.2618	$65.26	$65.25	$65.30
Purchase #7	$132.71	13.183%	$150.2052	$150.21	$150.20	$150.20
7 purchase totals =	$1,173.01		$1,302.8584	$1,302.85	$1,302.80	$1,302.80