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	$27.79	12.276%	$31.2015	$31.20	$31.20	$31.20
Purchase #2	$180.40	10.704%	$199.71	$199.71	$199.70	$199.70
Purchase #3	$133.83	12.517%	$150.5815	$150.58	$150.60	$150.60
Purchase #4	$85.55	5.152%	$89.9575	$89.96	$89.95	$90.00
Purchase #5	$154.37	8.220%	$167.0592	$167.06	$167.05	$167.10
Purchase #6	$132.97	4.816%	$139.3738	$139.37	$139.35	$139.40
Purchase #7	$37.91	6.313%	$40.3033	$40.30	$40.30	$40.30
7 purchase totals =	$752.82		$818.1868	$818.18	$818.15	$818.30