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	$250.83	5.267%	$264.0412	$264.04	$264.05	$264.00
Purchase #2	$98.71	9.242%	$107.8328	$107.83	$107.85	$107.80
Purchase #3	$248.69	12.576%	$279.9653	$279.97	$279.95	$280.00
Purchase #4	$84.91	5.102%	$89.2421	$89.24	$89.25	$89.20
Purchase #5	$63.61	12.332%	$71.4544	$71.45	$71.45	$71.50
Purchase #6	$131.47	11.515%	$146.6088	$146.61	$146.60	$146.60
Purchase #7	$17.51	13.490%	$19.8721	$19.87	$19.85	$19.90
7 purchase totals =	$895.73		$979.0167	$979.01	$979.00	$979.00