I don't shoot models, per-say, but I do a lot of portrait sessions on the side. Wide-open for faces or head/shoulder shots tends to blur out features here and there, so I usually go anywhere from f2.8-f4. For full body shots, you can sometimes go down to 1.8 for isolation, but even then I usually don't drop below 2.8. The further back I am, the lower f-stop I choose to keep the isolation.
I wouldn't go above f4 unless you really want a lot of the background in focus too, which will often detract from your "model".
My .02 as a semi-pro "hack".