Officially the rule is still "use whatever base the model was supplied with".
...Which can be a little weird, obviously, depending on whether you bought a Marine kit before or after 32mm bases became a thing. Hell, if you go far back enough even Terminators used to be supplied with 28mm bases.
At this point you're probably safe doing whatever "feels right" - within reason, of course.
I'm in the process of re-basing models as I touch up and work on my armies. For my Necrons this means switching all my Warriors to 32mm - they look so much better this way because on the old bases their feet were always hanging over the edges.
It didn't look right keeping other models on smaller bases so I decided to upgrade my Immortals, Deathmarks, etc. to 32mm as well - basically anything T4; for T5 models like Lychguard, Overlords, etc. they are getting bumped up to 40mm. I have to say the army as a whole looks a lot better for the changes.
Obviously this changes on a per-army basis. I'm upgrading power-armoured Space Marine minis to 32mm for similar reasons to my Necrons but I'm leaving Scout models on 28mm even though they're the same toughness - I find the base size looks just fine for them and it helps to emphasize that they have a little less 'presence' (and a weaker armour save) than fully-armoured Marines.
I think smaller, 'frailer' models like most Eldar, Guardsmen, Firewarriors, etc. still look fine on 28mm, but YMMV.
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