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	$5.73	11.712%	$6.4011	$6.40	$6.40	$6.40
Purchase #2	$64.53	12.470%	$72.5769	$72.58	$72.60	$72.60
Purchase #3	$23.77	8.726%	$25.8442	$25.84	$25.85	$25.80
Purchase #4	$90.57	7.490%	$97.3537	$97.35	$97.35	$97.40
Purchase #5	$96.52	6.541%	$102.8334	$102.83	$102.85	$102.80
Purchase #6	$111.81	10.590%	$123.6507	$123.65	$123.65	$123.70
Purchase #7	$137.27	10.282%	$151.3841	$151.38	$151.40	$151.40
7 purchase totals =	$530.20		$580.0441	$580.03	$580.10	$580.10