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	$122.69	11.155%	$136.3761	$136.38	$136.40	$136.40
Purchase #2	$176.71	5.412%	$186.2735	$186.27	$186.25	$186.30
Purchase #3	$100.93	11.439%	$112.4754	$112.48	$112.50	$112.50
Purchase #4	$34.26	13.309%	$38.8197	$38.82	$38.80	$38.80
Purchase #5	$123.40	5.129%	$129.7292	$129.73	$129.75	$129.70
Purchase #6	$77.45	7.489%	$83.2502	$83.25	$83.25	$83.30
Purchase #7	$144.32	5.510%	$152.272	$152.27	$152.25	$152.30
7 purchase totals =	$779.76		$839.1961	$839.20	$839.20	$839.30