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	$128.30	6.462%	$136.5907	$136.59	$136.60	$136.60
Purchase #2	$98.64	12.650%	$111.118	$111.12	$111.10	$111.10
Purchase #3	$164.98	11.133%	$183.3472	$183.35	$183.35	$183.30
Purchase #4	$72.59	13.422%	$82.333	$82.33	$82.35	$82.30
Purchase #5	$223.28	8.765%	$242.8505	$242.85	$242.85	$242.90
Purchase #6	$18.56	10.626%	$20.5322	$20.53	$20.55	$20.50
Purchase #7	$12.87	9.499%	$14.0925	$14.09	$14.10	$14.10
7 purchase totals =	$719.22		$790.8641	$790.86	$790.90	$790.80