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	$37.85	7.241%	$40.5907	$40.59	$40.60	$40.60
Purchase #2	$76.33	11.208%	$84.8851	$84.89	$84.90	$84.90
Purchase #3	$113.69	12.694%	$128.1218	$128.12	$128.10	$128.10
Purchase #4	$88.88	3.646%	$92.1206	$92.12	$92.10	$92.10
Purchase #5	$221.85	7.136%	$237.6812	$237.68	$237.70	$237.70
Purchase #6	$96.32	13.805%	$109.617	$109.62	$109.60	$109.60
Purchase #7	$139.97	4.570%	$146.3666	$146.37	$146.35	$146.40
7 purchase totals =	$774.89		$839.3830	$839.39	$839.35	$839.40