Liam1,
here is a link to the application guide
http://www.cic.gc.ca/english/pdf/kits/guides/3900E.pdf
The guide states when you need to fill out the Sponsor Evaluation IMM5481 and when you need to fill out the Financial Evaluation IMM5482.
Even if you do not have to fill out the Financial eval. you will still need to indicate in about 3 different places your income. One as you mentioned is your earnings in the 12 months preceeding the application (#3 IMM5481), there is another form where you indicate your work history for the last 5 years - including salaries (IMM5540) and also you are required to include a print out from your tax return (number 17 from the check list).
With that being said there is additional information you have to provide if you as the sponsor have residency outside of Canada when you apply. You have to provide proof that you will resume Canadian residency once your spouse receives there
PR Visa. #11 from IMM5540.
Now, if you are a Quebec resident (or plan to be) then I believe the minimum income requirement applies.
There is some conflicting information here....I would look over the guide and contact the CIC to get clarification.