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	$35.48	9.620%	$38.8932	$38.89	$38.90	$38.90
Purchase #2	$121.77	13.716%	$138.472	$138.47	$138.45	$138.50
Purchase #3	$64.66	4.166%	$67.3537	$67.35	$67.35	$67.40
Purchase #4	$80.41	10.391%	$88.7654	$88.77	$88.75	$88.80
Purchase #5	$16.54	13.818%	$18.8255	$18.83	$18.85	$18.80
Purchase #6	$39.38	4.303%	$41.0745	$41.07	$41.05	$41.10
Purchase #7	$28.27	4.138%	$29.4398	$29.44	$29.45	$29.40
7 purchase totals =	$386.51		$422.8241	$422.82	$422.80	$422.90