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	$15.87	13.507%	$18.0136	$18.01	$18.00	$18.00
Purchase #2	$196.69	4.536%	$205.6119	$205.61	$205.60	$205.60
Purchase #3	$139.91	13.812%	$159.2344	$159.23	$159.25	$159.20
Purchase #4	$170.78	4.713%	$178.8289	$178.83	$178.85	$178.80
Purchase #5	$150.57	11.781%	$168.3087	$168.31	$168.30	$168.30
Purchase #6	$94.44	3.297%	$97.5537	$97.55	$97.55	$97.60
Purchase #7	$43.78	12.652%	$49.319	$49.32	$49.30	$49.30
7 purchase totals =	$812.04		$876.8702	$876.86	$876.85	$876.80