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	$27.35	11.230%	$30.4214	$30.42	$30.40	$30.40
Purchase #2	$174.54	12.605%	$196.5408	$196.54	$196.55	$196.50
Purchase #3	$165.66	9.263%	$181.0051	$181.01	$181.00	$181.00
Purchase #4	$229.29	9.188%	$250.3572	$250.36	$250.35	$250.40
Purchase #5	$90.41	13.394%	$102.5195	$102.52	$102.50	$102.50
Purchase #6	$126.57	4.135%	$131.8037	$131.80	$131.80	$131.80
Purchase #7	$12.41	6.847%	$13.2597	$13.26	$13.25	$13.30
7 purchase totals =	$826.23		$905.9074	$905.91	$905.85	$905.90