I did my Rommel playtests in 6mm (partly because that is the scale of my desert stuff!).
All my bases (2mm, 6mm, 15mm and 20m) have a strip of magnetic tape on the rear edge, painted to blend in to the base. I can then use any markers I like (including strength points, formation affiliation markers ec) out of steel paper which stick to the mag strip.
It is less obtrusive and more flexible than unit labels. After all, rules come and go but figures last forever.
I’m too cheap to multibase 6mm stuff, I tried it for a while with my Corps level rules, but it just seemed such a waste and I broke them up again. Too much of a Yorkshireman. I do multibase 2mm though.
"Mistakes in the initial deployment cannot be rectified" - Helmuth von Moltke