Unless you have taken the IELTS before, you would have to guess your level. When you take the IELTS, you could get better or worse than what you guessed so if you have just enough points and your estimated English points are a big part of them, you better make sure you are not overestimating. If you are worried about it, you can take the IELTS now to give yourself proper points but if you are doing simplified application, do not send them the results until they ask.
You can see the guidelines for estimating your english on page 8 at http://www.cic.gc.ca/english/pdf/kits/guides/EG7.pdf