It depends on which stage of the processing you are and what part of the file you want to see.
If you are before SA (Sponsorship Approval), you should ask for the sponsor's file, because all there is in the principal applicants is that they are waiting for the decision in the sponsor's case. In that case if the sponsor is making the request there is no need for a consent form, because the sponsor is asking for his/her file.
If you want to see the principal applicant's (that's you
) file after you've got SA, and your sponsor is making the request, yes, you need to fill out the consent form. Because your spouse will ask for your file and you have to sign that you allow CIC to disclose your file to someone that isn't you.