It depends very much on what I'm trying to true up, and on which lathe.
On the Myford, or small Warco lookalike, I use a small diameter plunger set horizontally in the QD toolpost.
I also made a small spare chuck key, so I have the normal one at the front and the little one at the back of the 4 jaw.
It makes getting concentricity a doddle.
First do it by eye with the lines on the chuck face, then clock the workpiece next to jaw 1, zero the dial, clock @ jaw 3, move the workpiece half the difference.
Repeat for jaw 2 or 4 to the same reading, after one pass like that, it will be within a couple of thou, so one more run through will set you set up ready to go.
re mag bases, I have a couple of genuine M&W push button ones, as well as a clone.
All work equally well.
As my fingers are getting older and less strong, I find myself using the cheap imported ones with a rotary switch more often; they work well enough after the bases have been lapped on a bit of fine wet & dry.
For support arms, I use both the style you describe, as well as those with a couple of ball joints and a central knob. though they are a bit less rigid, and arguably better suited to lever, rather than plunger clocks.
Bill.