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	$16.92	6.423%	$18.0068	$18.01	$18.00	$18.00
Purchase #2	$53.97	10.309%	$59.5338	$59.53	$59.55	$59.50
Purchase #3	$130.65	5.407%	$137.7142	$137.71	$137.70	$137.70
Purchase #4	$38.57	9.528%	$42.2449	$42.24	$42.25	$42.20
Purchase #5	$225.48	11.208%	$250.7518	$250.75	$250.75	$250.80
Purchase #6	$93.64	7.554%	$100.7136	$100.71	$100.70	$100.70
Purchase #7	$167.56	11.475%	$186.7875	$186.79	$186.80	$186.80
7 purchase totals =	$726.79		$795.7526	$795.74	$795.75	$795.70