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	$19.92	6.536%	$21.222	$21.22	$21.20	$21.20
Purchase #2	$190.88	5.648%	$201.6609	$201.66	$201.65	$201.70
Purchase #3	$37.33	9.362%	$40.8248	$40.82	$40.80	$40.80
Purchase #4	$99.60	3.680%	$103.2653	$103.27	$103.25	$103.30
Purchase #5	$30.32	5.166%	$31.8863	$31.89	$31.90	$31.90
Purchase #6	$78.45	3.555%	$81.2389	$81.24	$81.25	$81.20
Purchase #7	$109.93	5.746%	$116.2466	$116.25	$116.25	$116.20
7 purchase totals =	$566.43		$596.3448	$596.35	$596.30	$596.30