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	$19.26	9.182%	$21.0285	$21.03	$21.05	$21.00
Purchase #2	$100.83	13.686%	$114.6296	$114.63	$114.65	$114.60
Purchase #3	$127.49	11.230%	$141.8071	$141.81	$141.80	$141.80
Purchase #4	$98.51	12.663%	$110.9843	$110.98	$111.00	$111.00
Purchase #5	$131.31	11.520%	$146.4369	$146.44	$146.45	$146.40
Purchase #6	$243.79	13.199%	$275.9678	$275.97	$275.95	$276.00
Purchase #7	$65.33	10.271%	$72.04	$72.04	$72.05	$72.00
7 purchase totals =	$786.52		$882.8942	$882.90	$882.95	$882.80