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	$18.46	9.462%	$20.2067	$20.21	$20.20	$20.20
Purchase #2	$158.50	6.285%	$168.4617	$168.46	$168.45	$168.50
Purchase #3	$18.57	9.565%	$20.3462	$20.35	$20.35	$20.30
Purchase #4	$155.82	12.872%	$175.8772	$175.88	$175.90	$175.90
Purchase #5	$46.82	11.805%	$52.3471	$52.35	$52.35	$52.30
Purchase #6	$50.61	8.494%	$54.9088	$54.91	$54.90	$54.90
Purchase #7	$81.80	8.822%	$89.0164	$89.02	$89.00	$89.00
7 purchase totals =	$530.58		$581.1641	$581.18	$581.15	$581.10