As of about four months ago I'm writing everything in a common universe.

[Note - I'm exhausted so hopefully this comment is not too full of typos.]

I'm doing this mainly because it gives me flexibility, which probably sounds odd. What I mean is, by explaining things in-story in terms of a common framework (or at least knowing what the in-universe explanation is, even if I don't explicitly lay it out), it allows me to connect my stories later on should I so choose. Knowing that's what I want to do, I can pay attention to when and where things happen up front so I don't have to retcon or outright break continuity later.

A common universe also means I don't have to screw around world building for every new story. I might have to come up with some new elements, but the foundation is already in place. Since I tend to waste time chasing minutia during world building, this saves me time.

I think of this as the Terry Pratchett model. Ignoring the first handful of Discworld books, what he ended up with was a bunch of loose series that followed certain groups of characters. The Witches, the Night Watch, Unseen University, etc. There were also stand-alone novels. Sometimes a story about the Witches (for example) would involve people from the Night Watch or the University, tying things together.

That sort of web of stories is what I'm going for, though my web will be much looser. The web approach seems necessary given I'm not that interested in writing continuous series covering a larger coherent arc---I do have some of those planned, but not many---but I do want to reuse characters, split off supporting characters and give them their own adventures, etc.

I'm also focused more on certain families and organizations than specific characters, and I think it's unlikely that any one character will get more than a handful of stories to star in. Most protagonists will be the star of just one story, though they'll probably turn up again elsewhere. I think of that approach as following the Redwall or Sackett model, though I won't hew to it as closely as Jacques or L'amour did.

And what's the point of not using a common universe, really? I have certain ideas about how the world is, how fantasy elements should work, and so on, and I'm always going to revert back to those ideas because they've been a lifetime in the making. Why not just say the stories are all connected? If I want a new playground, I can jump around in time, or shove my characters into a pocket dimension with different physical rules (though magic will always work the same way). One of my stories is set 300 years in the future, on a space station in a distant system. Several largely take place in other dimensions. One is on another planet, in the far future after space colonization, but following a technological collapse, and deals with the relations between normal people and people so magically altered they've become a new (fairly hostile and insular) species. Some of these stories are so wildly different you might not guess they take place in the same universe at all if you fail to pay attention to the details of the world mechanics.

If I ever absolutely need to write a story outside this common universe, that's fine; that's what I was doing the last several years anyway. But so far, I haven't yet run into a story idea that couldn't fit into it somehow.