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	$123.34	12.606%	$138.8882	$138.89	$138.90	$138.90
Purchase #2	$62.98	12.848%	$71.0717	$71.07	$71.05	$71.10
Purchase #3	$151.77	8.164%	$164.1605	$164.16	$164.15	$164.20
Purchase #4	$26.38	7.188%	$28.2762	$28.28	$28.30	$28.30
Purchase #5	$131.91	11.798%	$147.4727	$147.47	$147.45	$147.50
Purchase #6	$90.41	11.749%	$101.0323	$101.03	$101.05	$101.00
Purchase #7	$31.53	7.614%	$33.9307	$33.93	$33.95	$33.90
7 purchase totals =	$618.32		$684.8323	$684.83	$684.85	$684.90