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	$25.34	13.539%	$28.7708	$28.77	$28.75	$28.80
Purchase #2	$124.68	12.176%	$139.861	$139.86	$139.85	$139.90
Purchase #3	$123.46	12.726%	$139.1715	$139.17	$139.15	$139.20
Purchase #4	$62.77	9.598%	$68.7947	$68.79	$68.80	$68.80
Purchase #5	$84.37	7.784%	$90.9374	$90.94	$90.95	$90.90
Purchase #6	$100.54	8.442%	$109.0276	$109.03	$109.05	$109.00
Purchase #7	$109.60	10.668%	$121.2921	$121.29	$121.30	$121.30
7 purchase totals =	$630.76		$697.8551	$697.85	$697.85	$697.90