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	$120.87	5.484%	$127.4985	$127.50	$127.50	$127.50
Purchase #2	$221.71	8.186%	$239.8592	$239.86	$239.85	$239.90
Purchase #3	$75.38	6.560%	$80.3249	$80.32	$80.30	$80.30
Purchase #4	$201.26	12.500%	$226.4175	$226.42	$226.40	$226.40
Purchase #5	$81.55	9.419%	$89.2312	$89.23	$89.25	$89.20
Purchase #6	$43.80	3.739%	$45.4377	$45.44	$45.45	$45.40
Purchase #7	$29.79	4.300%	$31.071	$31.07	$31.05	$31.10
7 purchase totals =	$774.36		$839.8400	$839.84	$839.80	$839.80