No, that is not possible at this time (at least from my own experience). Currently, you can make changes on your iPhone, but in order for you to make changes to the same document on your Mac, you first have to:
-1) Download the file from your iCloud account
-2) Make the changes from Excel or Numbers app on Mac
-3) Save the document in Excel or Numbers
-4) Then, go back to your iCloud account online (on your Mac), and drag & drop the updated file to iCloud
I know I just said a mouthful, but I agree, that is a lot of extra steps for making a document sync on both your Mac and iPhone.... Hopefully Apple will have a solution for this in the future (where, for instance, a document can be accessed via iCloud directly from an iWork application (like Pages, Numbers, Keynote), and then we are able to make changes to the document, and as changes are made, it automatically updates in iCloud without us making any additional steps, if this makes any sense)....
Hope this helps....