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	$83.46	7.786%	$89.9582	$89.96	$89.95	$90.00
Purchase #2	$187.97	10.573%	$207.8441	$207.84	$207.85	$207.80
Purchase #3	$166.39	5.104%	$174.8825	$174.88	$174.90	$174.90
Purchase #4	$109.66	5.312%	$115.4851	$115.49	$115.50	$115.50
Purchase #5	$118.68	4.330%	$123.8188	$123.82	$123.80	$123.80
Purchase #6	$76.51	9.458%	$83.7463	$83.75	$83.75	$83.70
Purchase #7	$37.45	13.214%	$42.3986	$42.40	$42.40	$42.40
7 purchase totals =	$780.12		$838.1336	$838.14	$838.15	$838.10