What not to do with old/new pipes

Dear Heloise: There is no time to put grease or oils down a sink with old or new pipes! You pour off what you can as noted in a previous column, then wipe the pan with gently used paper napkins or towels that have been set aside. Throw the napkins away afterward. The little residue left on the pan is handled by detergent and hot water. You won’t have any more pipe worries this way.

Two asides: Do not throw floss in the toilet. If your pipes are old, the floss gets caught, builds up, and eventually causes a blockage. In the trash it goes! The second is to please ask your plumbing expert about pouring boiling water down the sink. I have read that either we can do this or should never do this, but no reasons have been given. So, I continue to be curious. To do or not to do? Thanks. — Emilie