Building a C&V mixte with stem shifters for the missus to tootle around town. I've yet to mess with stem shifters and am wondering how high or low on the stem does one go?

Set and cable them high then let her decide. Part of the fitting phase.

make sure to put anti-seize on the clamp screw threads.... they love to permanently rust in place..

I've got a similar set of Suntour stem shifters on one of my bikes, and they are set about like yours. Seems fine to me.
My missus has a couple of mixte's with downtube shift levers. She hasn't asked for the stem shifter mod, yet.

I love the Suntour shifters. I’ve tried them stem mounted but was not really happy no matter where I mounted them. I believe the problem for me is that they are mounted too far rearward from the axis of the stem. I believe that is the reason for the easy use of the great Schwinn Twin Stik levers as they are mounted forward of the stem axis. I think the forward mounting allows the hand to subconsciously bump into the handlebar stem and automatically find the levers. IIRC, I believe I once looked at the Suntours to see if they could be reversed to be positioned in front of the axis vs behind. I run them on down tubes now after trying stem mounts at several different heights. YMMV of course and with Suntour ratchets, you can’t go wrong.

Good point about the positioning of the lever axis being perhaps better in front of the stem quill, especially when a longer stem is used!

Suntour, for their part, actually offered some of their stem levers with a choice of different "pivot box" housings that mounted either in front of or behind the stem quill.
Recalling one such example, one of their earlier ratcheting stem lever assemblies having the larger, alloy levers, and usually also having short, thicker rubber covers on their tips, like these (though here, I converted them to an old 333 DT band clamp).



Another view of the same levers, mounted to a behind-the-quill version of the pivot/clamp housing, and which was a poor choice for the longer stem length:

For me I keep them just below the bars. On a really tall bike I was ridding I put them on the Top Tube just infront od the stem. That was a Sunrace set. I think it kinda makes a difference on the brand. Certainly the Suntour Ratchet stem shifters are the best.

Great insights and advice from the BF members as usual.

The higher the better. Because you WILL hit your knee on the shifters at some point.

It's been a while since I have mounted any stem shifters, but I recall putting them so the tip of the lever was just above the top of the stem. I tend shift both levers with one hand, so that makes it easier to reach across to the opposite side.