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	$27.50	13.687%	$31.2639	$31.26	$31.25	$31.30
Purchase #2	$154.74	9.516%	$169.4651	$169.47	$169.45	$169.50
Purchase #3	$38.81	11.401%	$43.2347	$43.23	$43.25	$43.20
Purchase #4	$142.34	8.177%	$153.9791	$153.98	$154.00	$154.00
Purchase #5	$83.88	7.469%	$90.145	$90.15	$90.15	$90.10
Purchase #6	$119.54	11.624%	$133.4353	$133.44	$133.45	$133.40
Purchase #7	$45.57	9.674%	$49.9784	$49.98	$50.00	$50.00
7 purchase totals =	$612.38		$671.5015	$671.51	$671.55	$671.50