Agree that it's all about what you're used to. Personally I can get a better and faster mitre with a grinder than a file but I don't assume this applies to everyone. And I am still at the paper template stage (not a first-timer, but nowhere near as experienced as a lot of people either). Although I don't bother with it for narrower tubes, like the ends of stays and bridge tubes, because I am starting to get a bit more of a sense of what the shape should look like.

As for mechanical mitring I am using that for one thing: joining steerers to fork legs (for a segmented or a unicrown fork). I made a fairly robust jig for holding the arbour from a cheap "tube mitring" thing in and I can get a pretty decent cut that way. The angles for one of these are very tricky and you can't wrap paper around a curved fork leg if it's a unicrown.