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	$16.37	12.284%	$18.3809	$18.38	$18.40	$18.40
Purchase #2	$185.30	7.147%	$198.5434	$198.54	$198.55	$198.50
Purchase #3	$233.47	11.462%	$260.2303	$260.23	$260.25	$260.20
Purchase #4	$92.57	11.623%	$103.3294	$103.33	$103.35	$103.30
Purchase #5	$84.75	5.310%	$89.2502	$89.25	$89.25	$89.30
Purchase #6	$51.59	6.629%	$55.0099	$55.01	$55.00	$55.00
Purchase #7	$103.25	7.123%	$110.6045	$110.60	$110.60	$110.60
7 purchase totals =	$767.30		$835.3486	$835.34	$835.40	$835.30