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	$13.89	9.227%	$15.1716	$15.17	$15.15	$15.20
Purchase #2	$125.75	13.136%	$142.2685	$142.27	$142.25	$142.30
Purchase #3	$80.90	12.385%	$90.9195	$90.92	$90.90	$90.90
Purchase #4	$22.50	10.572%	$24.8787	$24.88	$24.90	$24.90
Purchase #5	$102.93	10.192%	$113.4206	$113.42	$113.40	$113.40
Purchase #6	$177.63	3.699%	$184.2005	$184.20	$184.20	$184.20
Purchase #7	$108.92	8.774%	$118.4766	$118.48	$118.50	$118.50
7 purchase totals =	$632.52		$689.3360	$689.34	$689.30	$689.40