As others have said, sterilizing is not necessary as long as you give the bottles a good cleaning in warm water with soap. Make sure that the nipple is cleaned thoroughly. Most manufacturers recommend in the packaging of their products to sterilize before first use, which is what I did for my kids. It's just to get out any residue from the factory. Otherwise, I never sterilized my kids bottles.
Breast milk stays safe for much longer than formula, so if you are using breast milk you can leave it out for several hours without any harm coming to it. If you do end up using formula (I did for my kids) then you have to be very careful because formula does not last long without becoming harmful, because bacteria build up quickly, so in that case you will need to wash bottles more frequently.
So as a general answer, there is no need to sterilize, just do a good washing in between uses.