Most purses have multiple pockets, so I just put the iPhone in one of them by itself. I use either the Ifrogz Luxe, Luxe Lean or Speck Fitted with no screen protector. The screen is still perfect with no scratches.
I think as long as you keep it separate from things like keys and coins you should be ok. I keep my purse clean inside too, so no sand or anything is in it.