Would be great to hear why, Paul.
If you think it is "standard practice" offer us a link from the other person in the world who's shared that.
Talc helps that last bit of bead get over the rim edge: really this is mostly for a new tyre, and only needed on the last 30 degree arc.
How is talc on a tube beneficial? The tube will go where it's put. Might need a poke at the end to make sure it's not interfering with the final thumb push.
An unnecessary remark which I don't intend to dignify with a link or citation. It's a handy cycling tip, nothing more important. In hindsight "standard" was a poor choice but I certainly think "common practice" is appropriate. It's something I've done for 25 or more years. It's common practice most of my regular cycling buddies and makes roadside repairs much easier.
Talc can make tight fitting items easier to fit, "surgical" style** gloves are often talced by the manufacturer. Tyres are easier to get on to a wheel if talc is applied to them. The logic behind talcing a spare tube is this. When the tube is inserted into the tyre some of the talc transfers on to the tyre. It also gets on the rider's hands and transfers to the tyre. One then has a talced tyre which is much easier to slip on than one which hasn't been talced. Probably a bit messy with tubeless. The alternative is to carry some Johnson's baby powder in one's kit.
** for clarity I say "style" as I imagine actual surgical gloves may not use talc. Possibly a safety issue?
