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	$19.43	11.356%	$21.6365	$21.64	$21.65	$21.60
Purchase #2	$89.27	10.760%	$98.8755	$98.88	$98.90	$98.90
Purchase #3	$224.63	10.523%	$248.2678	$248.27	$248.25	$248.30
Purchase #4	$16.64	6.472%	$17.7169	$17.72	$17.70	$17.70
Purchase #5	$143.57	5.213%	$151.0543	$151.05	$151.05	$151.10
Purchase #6	$73.97	9.795%	$81.2154	$81.22	$81.20	$81.20
Purchase #7	$19.72	6.751%	$21.0513	$21.05	$21.05	$21.10
7 purchase totals =	$587.23		$639.8177	$639.83	$639.80	$639.90