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	$16.59	10.854%	$18.3907	$18.39	$18.40	$18.40
Purchase #2	$113.94	11.754%	$127.3325	$127.33	$127.35	$127.30
Purchase #3	$150.85	10.612%	$166.8582	$166.86	$166.85	$166.90
Purchase #4	$250.93	12.490%	$282.2712	$282.27	$282.25	$282.30
Purchase #5	$123.79	10.216%	$136.4364	$136.44	$136.45	$136.40
Purchase #6	$19.55	6.648%	$20.8497	$20.85	$20.85	$20.80
Purchase #7	$92.60	12.729%	$104.3871	$104.39	$104.40	$104.40
7 purchase totals =	$768.25		$856.5258	$856.53	$856.55	$856.50