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	$19.26	9.648%	$21.1182	$21.12	$21.10	$21.10
Purchase #2	$93.30	10.789%	$103.3661	$103.37	$103.35	$103.40
Purchase #3	$158.74	7.211%	$170.1867	$170.19	$170.20	$170.20
Purchase #4	$181.41	5.746%	$191.8338	$191.83	$191.85	$191.80
Purchase #5	$85.44	4.282%	$89.0985	$89.10	$89.10	$89.10
Purchase #6	$146.77	13.558%	$166.6691	$166.67	$166.65	$166.70
Purchase #7	$133.42	12.813%	$150.5151	$150.52	$150.50	$150.50
7 purchase totals =	$818.34		$892.7875	$892.80	$892.75	$892.80