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	$35.85	10.290%	$39.539	$39.54	$39.55	$39.50
Purchase #2	$76.80	11.748%	$85.8225	$85.82	$85.80	$85.80
Purchase #3	$79.25	5.352%	$83.4915	$83.49	$83.50	$83.50
Purchase #4	$62.89	3.736%	$65.2396	$65.24	$65.25	$65.20
Purchase #5	$144.43	4.234%	$150.5452	$150.55	$150.55	$150.50
Purchase #6	$165.95	13.304%	$188.028	$188.03	$188.05	$188.00
Purchase #7	$77.79	4.694%	$81.4415	$81.44	$81.45	$81.40
7 purchase totals =	$642.96		$694.1073	$694.11	$694.15	$693.90