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	$243.51	12.123%	$273.0307	$273.03	$273.05	$273.00
Purchase #2	$180.51	13.718%	$205.2724	$205.27	$205.25	$205.30
Purchase #3	$201.29	8.742%	$218.8868	$218.89	$218.90	$218.90
Purchase #4	$167.59	10.290%	$184.835	$184.84	$184.85	$184.80
Purchase #5	$211.82	12.601%	$238.5114	$238.51	$238.50	$238.50
Purchase #6	$100.98	12.370%	$113.4712	$113.47	$113.45	$113.50
Purchase #7	$53.45	5.206%	$56.2326	$56.23	$56.25	$56.20
7 purchase totals =	$1,159.15		$1,290.2401	$1,290.24	$1,290.25	$1,290.20