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	$212.83	12.540%	$239.5189	$239.52	$239.50	$239.50
Purchase #2	$139.63	9.665%	$153.1252	$153.13	$153.15	$153.10
Purchase #3	$35.51	4.116%	$36.9716	$36.97	$36.95	$37.00
Purchase #4	$142.31	7.170%	$152.5136	$152.51	$152.50	$152.50
Purchase #5	$83.95	6.582%	$89.4756	$89.48	$89.50	$89.50
Purchase #6	$92.35	11.343%	$102.8253	$102.83	$102.85	$102.80
Purchase #7	$58.91	12.239%	$66.12	$66.12	$66.10	$66.10
7 purchase totals =	$765.49		$840.5502	$840.56	$840.55	$840.50