Well, turning your phone off doesn't, in an of itself, affect how long a battery lasts... but there are a couple of bad practices.... and neither is all that likely to be honest. If you store a LiPO battery for an extended amount of time, they don't like to be either fully charged or fully discharged. They really should be about 50-70%. That's why whenever you buy something with a battery, they come half-charged. It isn't them being cheap and saving the extra pennies to fully charge, it is just good practice.
But that's only going to come into play if you plan on having your phone sit there for a month or two... which I suspect isn't the case with anyone here anyhow.