Po Required Precancel Gum Removal?

I know I had heard of the rule but I can't remember where I first saw it.

Steve Wittig is a major collector /authority, so I would actually trust him on this.

He's been on ebay trying to dispose of his collection (or parts of it) for as long as I've been collecting US stamps (um, almost a year); I don't know if anything much sells, but just looking at the individual auctions, his material must be vast ...

On the other hand, my online search only reveals that pre-war the PO refused to allow collectors to buy precancels; no mention of them being allowed to have gum-free ones. PO apparently relented somewhat when they found that collectors were getting gummed copies from companies (overstocks, etc.). So I wonder whether the rule was really as clear as SW says.