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	$29.38	6.303%	$31.2318	$31.23	$31.25	$31.20
Purchase #2	$117.41	10.170%	$129.3506	$129.35	$129.35	$129.40
Purchase #3	$28.64	13.333%	$32.4586	$32.46	$32.45	$32.50
Purchase #4	$138.41	11.495%	$154.3202	$154.32	$154.30	$154.30
Purchase #5	$82.71	8.153%	$89.4533	$89.45	$89.45	$89.50
Purchase #6	$50.48	10.665%	$55.8637	$55.86	$55.85	$55.90
Purchase #7	$44.86	7.718%	$48.3223	$48.32	$48.30	$48.30
7 purchase totals =	$491.89		$541.0005	$540.99	$540.95	$541.10