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	$83.35	7.127%	$89.2904	$89.29	$89.30	$89.30
Purchase #2	$167.38	6.412%	$178.1124	$178.11	$178.10	$178.10
Purchase #3	$163.97	13.168%	$185.5616	$185.56	$185.55	$185.60
Purchase #4	$136.49	13.478%	$154.8861	$154.89	$154.90	$154.90
Purchase #5	$85.60	6.609%	$91.2573	$91.26	$91.25	$91.30
Purchase #6	$103.64	8.573%	$112.5251	$112.53	$112.55	$112.50
Purchase #7	$27.40	9.167%	$29.9118	$29.91	$29.90	$29.90
7 purchase totals =	$767.83		$841.5447	$841.55	$841.55	$841.60