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	$19.32	8.834%	$21.0267	$21.03	$21.05	$21.00
Purchase #2	$232.25	7.380%	$249.3901	$249.39	$249.40	$249.40
Purchase #3	$41.93	8.328%	$45.4219	$45.42	$45.40	$45.40
Purchase #4	$241.77	6.680%	$257.9202	$257.92	$257.90	$257.90
Purchase #5	$249.98	11.811%	$279.5051	$279.51	$279.50	$279.50
Purchase #6	$39.74	13.385%	$45.0592	$45.06	$45.05	$45.10
Purchase #7	$49.77	11.725%	$55.6055	$55.61	$55.60	$55.60
7 purchase totals =	$874.76		$953.9287	$953.94	$953.90	$953.90