Why do you think you will lose your 0.25% everytime due to valuation? I believe that either you are offering much more than the market value to secure a property or putting your hands into the OTP territory.
All of my properties were valued on the contract price and I guess other SSers might have a similar experience.
Do the numbers again. Check with a mortgage broker the max you can borrow taking into account the stamp duty, legal fees, etc. Only commit yourself to what you can borrow and do not offer way high than 'market value'.
The agents will keep saying that the market is hot and what not. You put forth your offer with the clause, get the pest & building inspection done and valuation done.
You will be alright. Pm me if you need contact details of my Mortgage Broker. I have been pretty happy with him thus far.