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	$5.46	12.710%	$6.154	$6.15	$6.15	$6.20
Purchase #2	$178.33	11.478%	$198.7987	$198.80	$198.80	$198.80
Purchase #3	$41.94	8.709%	$45.5926	$45.59	$45.60	$45.60
Purchase #4	$173.48	12.804%	$195.6924	$195.69	$195.70	$195.70
Purchase #5	$43.57	4.513%	$45.5363	$45.54	$45.55	$45.50
Purchase #6	$68.56	5.479%	$72.3164	$72.32	$72.30	$72.30
Purchase #7	$66.72	11.140%	$74.1526	$74.15	$74.15	$74.20
7 purchase totals =	$578.06		$638.2430	$638.24	$638.25	$638.30