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	$29.88	8.363%	$32.3789	$32.38	$32.40	$32.40
Purchase #2	$78.59	12.188%	$88.1685	$88.17	$88.15	$88.20
Purchase #3	$239.49	10.558%	$264.7754	$264.78	$264.80	$264.80
Purchase #4	$242.94	12.370%	$272.9917	$272.99	$273.00	$273.00
Purchase #5	$116.71	9.151%	$127.3901	$127.39	$127.40	$127.40
Purchase #6	$43.57	3.234%	$44.9791	$44.98	$45.00	$45.00
Purchase #7	$85.56	4.395%	$89.3204	$89.32	$89.30	$89.30
7 purchase totals =	$836.74		$920.0041	$920.01	$920.05	$920.10