I don't think I have ever used the sleep button to do this. 100% of the time since day 1 with the original iphone and on my 3g, I have used the home button.
Now see I put my phone in my right pocket, and whenever I put it away I put it top down (home button up) so whenever I take it out I directly put my thumb on the home button, so it's the quickest way to turn it on.
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