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	$5.99	11.155%	$6.6582	$6.66	$6.65	$6.70
Purchase #2	$215.60	4.299%	$224.8686	$224.87	$224.85	$224.90
Purchase #3	$90.61	11.346%	$100.8906	$100.89	$100.90	$100.90
Purchase #4	$161.80	4.369%	$168.869	$168.87	$168.85	$168.90
Purchase #5	$77.47	12.667%	$87.2831	$87.28	$87.30	$87.30
Purchase #6	$224.48	4.620%	$234.851	$234.85	$234.85	$234.90
Purchase #7	$6.31	13.808%	$7.1813	$7.18	$7.20	$7.20
7 purchase totals =	$782.26		$830.6018	$830.60	$830.60	$830.80