Whenever I've boiled my kettle and there is leftover water, I pour it down the kitchen drain. It serves two purposes: I prevent hardwater buildup in my kettle and I help clear the drain of any grease, since the hot water will take it down. I've lived in my house for 9 years now and I've never had to use drain cleaner on the kitchen sink. Bathtub, yes, because of my hair but never the kitchen. Now that we have green bin (organics) recycling, I just let my grease harden and then scrape it into the bin.