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	$25.83	6.412%	$27.4862	$27.49	$27.50	$27.50
Purchase #2	$146.36	3.640%	$151.6875	$151.69	$151.70	$151.70
Purchase #3	$43.53	12.737%	$49.0744	$49.07	$49.05	$49.10
Purchase #4	$145.44	5.793%	$153.8653	$153.87	$153.85	$153.90
Purchase #5	$229.93	12.712%	$259.1587	$259.16	$259.15	$259.20
Purchase #6	$31.31	7.248%	$33.5793	$33.58	$33.60	$33.60
Purchase #7	$103.41	10.404%	$114.1688	$114.17	$114.15	$114.20
7 purchase totals =	$725.81		$789.0202	$789.03	$789.00	$789.20