The lock matters most during the gap between an accepted offer and possession, which is exactly the window a financing condition is designed to cover. The rate a lender locks in is also the rate your file gets tested against, using the same roughly 39 percent housing cost and 44 percent total debt ratios CMHC applies at approval.
Without a lock, a rate increase before closing can change what you qualify for on the same property, which is a bigger problem than simply paying more, since it can push a buyer's debt ratios past what underwriting allows.
Ask your lender exactly what your rate lock covers and for how long before you rely on it. Get your hand-reviewed valuation from Joel Dyck.