@dpierre This is really 2 questions: Do you think you can guess future interest rates better than the bank? Can you afford to be wrong?
Over the long term, variable rates tend to be lower than fixed rates because you assume the risk rather than the bank, as with all financial products, higher risk profiles lead to a likelihood of higher rewards.
If you can afford that risk, you're better off in most cases staying variable. The only real exception is if you have some reason to think you can predict future rates better than the many specialists the bank has hired for exactly that purpose.
That said, if you can't afford the risk, then locking in will guarantee your rate, and make it a lot easier to plan around, at least until renewal. Remember that the locked in rate will only last until renewal, and at that time you will be vulnerable again, just like if you were on a variable rate.