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	$17.90	11.816%	$20.0151	$20.02	$20.00	$20.00
Purchase #2	$215.61	11.447%	$240.2909	$240.29	$240.30	$240.30
Purchase #3	$45.50	10.622%	$50.333	$50.33	$50.35	$50.30
Purchase #4	$114.33	4.550%	$119.532	$119.53	$119.55	$119.50
Purchase #5	$142.95	11.168%	$158.9147	$158.91	$158.90	$158.90
Purchase #6	$41.89	3.261%	$43.256	$43.26	$43.25	$43.30
Purchase #7	$225.88	5.380%	$238.0323	$238.03	$238.05	$238.00
7 purchase totals =	$804.06		$870.3740	$870.37	$870.40	$870.30