I always keep everything up to date, I have auto-updates enabled for the core, themes and plugins.
To be fair, it's easy for me as I don't have too many plugins on a typical WP website, but I can understand how tedious it can be if you do.
The security risk may not seem high, but if something happens it may result in data loss, corruption of the whole system where you have other websites, etc.
So however small the risk may seem, it's still there.
WP itself is perhaps the most backward-compatible software out there, and for the most part you can find an alternative plugin or theme that's more established and follows similar compatibility guidelines. Not always, but for most of the time. So if you do that and enable auto-updates for them, you'll be left with just a couple to manually check.
A good practice is to have a staging or even a local website where you can test your updates without having to worry about the consequences. Once everything is fine, you can do it on the actual website. Having and managing a staging website may seem a lot of work, but it eliminates a lot of work you'd have to do otherwise every once in a while, plus it lowers security risks, worth it for a long run. And as mentioned above by baldidiot, today hosting providers make it quite easy.
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