This can't be a to-die-on rule though. Retail would've never bought GOOG, or TSLA, or AAPL if that were the case. Maybe I'm just being pedantic.
Even for good assets there's a price you shouldn't pay. People are joking(?) about triple-layer SPVs where you can get pre-IPO exposure but at higher-than-IPO price.
Google and Apple didn't go through ten funding rounds like today's startups do. Apple had one angel and three rounds, Google had one angel and literally just an A round after that; then retail investors could capture all the upside. Now there's way more time for private investors to pick the bones clean before it gets dumped on the public.