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	$45.74	12.585%	$51.4964	$51.50	$51.50	$51.50
Purchase #2	$203.49	10.784%	$225.4344	$225.43	$225.45	$225.40
Purchase #3	$118.65	12.162%	$133.0802	$133.08	$133.10	$133.10
Purchase #4	$40.30	3.669%	$41.7786	$41.78	$41.80	$41.80
Purchase #5	$188.47	13.632%	$214.1622	$214.16	$214.15	$214.20
Purchase #6	$83.83	6.749%	$89.4877	$89.49	$89.50	$89.50
Purchase #7	$57.42	11.671%	$64.1215	$64.12	$64.10	$64.10
7 purchase totals =	$737.90		$819.5610	$819.56	$819.60	$819.60