For OS updates, I wait until the images are hosted by Google. Whether it gets hosted before I receive the new OS update notification or after is not relevant.
For Google apps, I update.
For 3rd party apps, it's a case by case basis.
If it's a major company like Amazon, Netflix, eBay, I update.
If it's an indie developer that I trust(Cerberus, SafeInCloud, Nova Launcher, Greenify, Duolingo, Pixel Dungeon, Kairosoft games, MX Player), I update.
All other 3rd party apps, I wait a bit and always read reviews in case they switch to a lousy UI, remove features, or add completely useless features like social integration.
Prime example: TuneIn Radio. I'm still on v11.3 and have refused to upgrade for the past 6 months+. Screw their new UI, creating new mandatory accounts, and their bullshit Facebook/Twitter/whatever social network integration.