I recently read that for cakes you should not substitute margarine for butter, the results will not be the same. You can do it for cookies and things like that but for a cake it should be butter....
I would not use marg in cookie recipes unless you want them to spread like pancakes!
I ALWAYS use margarine and the cookies don't over-spread. Actually, sometimes they don't spread enough and come out like little half balls! 
I was in my late teens before I knew that the stuff I grew up on and that we called "butter" wasn't "butter". I've never purchased or used "butter" in my entire life. I personally think real "butter" tastes nasty and wouldn't eat it if my life depended on it.
Butter and margarine are interchangeable in most recipes. 
If your recipe calls for unsalted butter and you want to substitute margarine, then just don't add the additional salt called for...
Everything you ever wanted to know about the butter vs. margarine issue:
(This is right up there with the scratch vs. mix debates.)
http://cakecentral.com/cake-decorating-ftopict-593168-.html
HTH
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