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	$36.25	10.285%	$39.9783	$39.98	$40.00	$40.00
Purchase #2	$82.79	12.747%	$93.3432	$93.34	$93.35	$93.30
Purchase #3	$174.30	5.626%	$184.1061	$184.11	$184.10	$184.10
Purchase #4	$215.85	10.677%	$238.8963	$238.90	$238.90	$238.90
Purchase #5	$97.92	11.772%	$109.4471	$109.45	$109.45	$109.40
Purchase #6	$34.31	7.868%	$37.0095	$37.01	$37.00	$37.00
Purchase #7	$37.72	12.622%	$42.481	$42.48	$42.50	$42.50
7 purchase totals =	$679.14		$745.2615	$745.27	$745.30	$745.20