As per new directions of CIC, the validity period of IELTS result for Canada Immigration is 2 years. For instance, if you have taken the test on 2 January 2012 than it will valid till 01 January 2014. In addition, the validity of IELTS itself is 2 years. So keeping in mind the validity period of IELTS test and your immigration case processing time you can decide the date of taking the IELTS test.
It is also recommended that dont wait for the time when CHC demand documents to finalize your immigration case. But try to well prepare IELTS and take it time before the expected time of Immigration.
As per the current procedure of Canadian Immigration, you have to submit your IELTS result along with your Canadian Immigration Application.
IELTS scores has also been reduced for Immigration. Look at the following article to see latest changes.