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	$29.40	8.182%	$31.8055	$31.81	$31.80	$31.80
Purchase #2	$29.97	4.702%	$31.3792	$31.38	$31.40	$31.40
Purchase #3	$166.45	10.155%	$183.353	$183.35	$183.35	$183.40
Purchase #4	$152.28	8.128%	$164.6573	$164.66	$164.65	$164.70
Purchase #5	$102.60	10.650%	$113.5269	$113.53	$113.55	$113.50
Purchase #6	$31.33	12.588%	$35.2738	$35.27	$35.25	$35.30
Purchase #7	$204.92	4.347%	$213.8279	$213.83	$213.85	$213.80
7 purchase totals =	$716.95		$773.8236	$773.83	$773.85	$773.90