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	$37.89	12.109%	$42.4781	$42.48	$42.50	$42.50
Purchase #2	$133.55	12.218%	$149.8671	$149.87	$149.85	$149.90
Purchase #3	$81.82	10.504%	$90.4144	$90.41	$90.40	$90.40
Purchase #4	$125.81	12.264%	$141.2393	$141.24	$141.25	$141.20
Purchase #5	$217.71	7.712%	$234.4998	$234.50	$234.50	$234.50
Purchase #6	$83.68	5.764%	$88.5033	$88.50	$88.50	$88.50
Purchase #7	$207.69	13.520%	$235.7697	$235.77	$235.75	$235.80
7 purchase totals =	$888.15		$982.7717	$982.77	$982.75	$982.80