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	$18.91	13.569%	$21.4759	$21.48	$21.50	$21.50
Purchase #2	$113.84	11.237%	$126.6322	$126.63	$126.65	$126.60
Purchase #3	$129.46	7.590%	$139.286	$139.29	$139.30	$139.30
Purchase #4	$182.34	7.241%	$195.5432	$195.54	$195.55	$195.50
Purchase #5	$49.61	7.614%	$53.3873	$53.39	$53.40	$53.40
Purchase #6	$15.99	13.496%	$18.148	$18.15	$18.15	$18.10
Purchase #7	$243.83	13.161%	$275.9205	$275.92	$275.90	$275.90
7 purchase totals =	$753.98		$830.3931	$830.40	$830.45	$830.30