tldr: there's been some research into it – turns out that if you have a few parts (instruments), the randomness you add to each of them needs to be correlated:
> "The first [example] has had completely random timing errors inserted [...] with no link between the errors in different parts. The result sounds unmistakably unmusical and inhuman."
someone built a "humanizing" plugin for Ableton Live based on that research.
here's some quotes i liked:
> "[...] the timing of each individual note is dependent on every single note that both players had already played – a minor timing hiccup near the start of a piece will continue to affect every single note after it, up to the last notes. And when you play a duet every note your partner plays affects your playing, and every note you play affects your partner [...]"
> "[...] if everything is recorded together in the same take then quite large variations in timing are no problem – they don't sound like errors, just the natural movement of the music. But if the parts are multi-tracked, or sequenced parts are mixed with human parts, then the timing errors are glaringly obvious, they sound wrong because they are unnatural, and our capability to identify the uncanny marks them out as unpleasant and undesirable [...]"