Put it under contract THEN do you due diligence

[Dean Julie]

I have heard this tactic/strategy/philosophy/(BS) taught and suggested by many. The theory is, if the property turns out to really suck, you just renig on the contract (with some contingency) and you're on your merry way. However, when I've went back to these people that said it and offer them deals, all of a sudden, they want to do miles and miles of due diligence before they even get close to considering putting properties under contract.
So, those that really do do real-estate, what do you do? Do you do your due diligence first? Or do you put it under contract first?

Dean