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	$130.58	6.687%	$139.3119	$139.31	$139.30	$139.30
Purchase #2	$9.51	5.403%	$10.0238	$10.02	$10.00	$10.00
Purchase #3	$209.54	7.540%	$225.3393	$225.34	$225.35	$225.30
Purchase #4	$219.92	10.309%	$242.5916	$242.59	$242.60	$242.60
Purchase #5	$160.25	5.689%	$169.3666	$169.37	$169.35	$169.40
Purchase #6	$78.76	3.391%	$81.4308	$81.43	$81.45	$81.40
Purchase #7	$39.28	6.500%	$41.8332	$41.83	$41.85	$41.80
7 purchase totals =	$847.84		$909.8972	$909.89	$909.90	$909.80