I�m not a Rov or an accountant so this isn�t halachic or professional advice- just my personal opinion.
Signing your name to something that you know isn�t true, even if there�s no way you�re going to be caught is just wrong, plane and simple- there�s no two ways about it.
Being a citizen of a country entitles one to certain privileges and protections which ultimately costs the country money. While you may feel that paying taxes for the same income to the Israeli Government and then to the US Government is double paying- the respective governments look at us as their share of the costs to look out for their citizens.
Who decides what�s fair and unfair?
If your clients in Israel don�t feel like paying US income taxes, why don�t they just denounce their citizenship?