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	$111.88	11.386%	$124.6187	$124.62	$124.60	$124.60
Purchase #2	$40.26	5.157%	$42.3362	$42.34	$42.35	$42.30
Purchase #3	$147.73	9.418%	$161.6432	$161.64	$161.65	$161.60
Purchase #4	$36.34	4.542%	$37.9906	$37.99	$38.00	$38.00
Purchase #5	$31.42	6.640%	$33.5063	$33.51	$33.50	$33.50
Purchase #6	$86.79	4.656%	$90.8309	$90.83	$90.85	$90.80
Purchase #7	$142.51	4.276%	$148.6037	$148.60	$148.60	$148.60
7 purchase totals =	$596.93		$639.5296	$639.53	$639.55	$639.40