Good vacuum practice and automobile practice and refrigeration practice says to grease the "O" rings with a compatible fluid/grease. Why does plumbing have to be different? I know the rubber washer is flat, but the principle applies. As you tighten a non lubercated fitting, there is a tendency for the washer to get distorted. Grease prevents that process.
The silicon grease on the threads assures easy removal at a later time.
Not required. Just helpful
Try this test:Take a trap and grease the threads. Take another trap and don't grease the threads. Wait 20 years. Which trap will be easier to remove?
Silicon grease does no harm. In fact, it makes the next plumber's job potentially easier.
Note here prevents the rubber from drying out: Silicone Grease and Compounds
and this one: Select Products Plumbers Grease
Plumbers Grease Lubricates & Protects metal, rubber & Plastic parts. (Use on "O" rings & value stems)
Note the word metal which the P trap is made of.
Sorry Tom, I still disagree. Hopefully I've provided proof that it's a "GOOD IDEA". If
easy removal is a good thing. If
lubercation makes easy removal. If
silicon grease is a good lubercant. If
silicon grease is compatible with water systems. If
silicon grease prevents rubber/"O" rings/gaskets from drying out. If
a metal P trap contains a rubber gasket. If
a metal P trap is made of metal Then silicon grease can be used on metal "P" traps to prevent gaskets from drying out and also the facilitate removal of the fittings at a later date