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	$19.82	5.858%	$20.9811	$20.98	$21.00	$21.00
Purchase #2	$91.26	3.625%	$94.5682	$94.57	$94.55	$94.60
Purchase #3	$151.90	13.138%	$171.8566	$171.86	$171.85	$171.90
Purchase #4	$63.36	7.575%	$68.1595	$68.16	$68.15	$68.20
Purchase #5	$54.41	13.287%	$61.6395	$61.64	$61.65	$61.60
Purchase #6	$106.73	11.862%	$119.3903	$119.39	$119.40	$119.40
Purchase #7	$146.79	4.161%	$152.8979	$152.90	$152.90	$152.90
7 purchase totals =	$634.27		$689.4931	$689.50	$689.50	$689.60