Getting pre‑approved for a mortgage before looking at properties gives you a more realistic expectation of what you can afford.  It also helps your Realtor narrow the field and only show you properties that will work in your financial situation.   However, keep in mind that the pre‑approved amounts can overestimate what you can actually afford to pay. 
Pre‑approval does not guarantee you will be approved once you actually apply if market conditions, interest rates, or your personal circumstances change.  We have some great mortgage brokers that have intimate knowledge of the Whistler market - if you would like a referral please give me a call.