Why ever? This is a practice that dates back to a time when moms made formula from evaporated milk and had to boil the milk to make it digestible.City water supplies were also inconsistent. Today it is a waste of time unless you are unsure of your water.A good bottle brush, and hot soapy water will work well for both.
Wash them thoroughly. You can simply wash them well (i advise using tools dedicated to washing the nipples and bottles) then let them air dry and store them soon after. You can also use sterilizer bags for the microwave. They are quick and easy and do the job well.
Hot water and soap. You can wash with hot water and soap. You can further sterilize by boiling a pot of water and placing the nipples and pacifiers in the water at a rolling boil for 3-5 minutes and then letting them cool and dry.