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	$120.45	13.660%	$136.9035	$136.90	$136.90	$136.90
Purchase #2	$209.49	13.684%	$238.1566	$238.16	$238.15	$238.20
Purchase #3	$107.80	13.404%	$122.2495	$122.25	$122.25	$122.20
Purchase #4	$113.42	8.560%	$123.1288	$123.13	$123.15	$123.10
Purchase #5	$109.88	9.123%	$119.9044	$119.90	$119.90	$119.90
Purchase #6	$29.57	3.532%	$30.6144	$30.61	$30.60	$30.60
Purchase #7	$31.80	4.765%	$33.3153	$33.32	$33.30	$33.30
7 purchase totals =	$722.41		$804.2725	$804.27	$804.25	$804.20