As babies do not have a fully developed immune system yet, we would highly recommend sterilizing the bottles and keeping them in a clean, sterile environment for the first 6 months of life. Doing so will prevent a build-up of germs and bacteria, which may cause your baby to be more susceptible to falling sick.
Once the bottles are sterilized, do avoid leaving them in the sterilizer as repetitive heat treatment to the plastic parts may cause some premature aging of the product.