What are the "best practices" for adding custom buttons these days,
and what are the reasons?

I need only two buttons, and, because the toolbar would be so small,
it has to share a row with another toolbar or else it takes up too
much space. Because the Standard toolbar constantly changes size
depending on the explorer view, it gets bumped around quite a bit and
looks stranded. I also have to remember locations, which is
especially noisome on inspector windows.

Adding the buttons to the Standard toolbar seems to be ill-regarded
but definitely solves these aesthetic problems. In my limited
experience, the rule of thumb would be: large number of buttons/
controls, make your own, small number of buttons, add to Standard.

Any feedback would be appreciated.