i sterilise everything. I wash in hot soapy water. Rinse in warm water, and then sterlise in avent steamer microwave one. As soon as it comes out of steamer, i fill with water to required amount, and then seal...
I will stop sterlising in the avent thing at 6months, or, as soon as he starts putting stuff in his mouth.

ETA - the health nurse told me no need to sterlise, just wash in hot soapy water, and rinse in hot water.