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	$18.69	12.735%	$21.0702	$21.07	$21.05	$21.10
Purchase #2	$216.47	3.423%	$223.8798	$223.88	$223.90	$223.90
Purchase #3	$121.42	12.182%	$136.2114	$136.21	$136.20	$136.20
Purchase #4	$161.42	6.430%	$171.7993	$171.80	$171.80	$171.80
Purchase #5	$75.79	7.636%	$81.5773	$81.58	$81.60	$81.60
Purchase #6	$140.33	12.630%	$158.0537	$158.05	$158.05	$158.10
Purchase #7	$33.34	13.606%	$37.8762	$37.88	$37.90	$37.90
7 purchase totals =	$767.46		$830.4679	$830.47	$830.50	$830.60