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	$50.57	8.723%	$54.9812	$54.98	$55.00	$55.00
Purchase #2	$40.47	11.471%	$45.1123	$45.11	$45.10	$45.10
Purchase #3	$103.73	9.299%	$113.3759	$113.38	$113.40	$113.40
Purchase #4	$163.87	4.652%	$171.4932	$171.49	$171.50	$171.50
Purchase #5	$114.66	8.115%	$123.9647	$123.96	$123.95	$124.00
Purchase #6	$123.47	10.623%	$136.5862	$136.59	$136.60	$136.60
Purchase #7	$18.31	13.709%	$20.8201	$20.82	$20.80	$20.80
7 purchase totals =	$615.08		$666.3336	$666.33	$666.35	$666.40