I haven't had a problem with the plastic Valks on the
GW stands, they're pretty stable. I haven't tried putting a
FW Valk on one as I don't want to go hacking a cross into the underside of the resin, and haven't gotten around to making a cross slot piece to pin to on like the plastic kit has. They should be a lot more stable than my homemade stands - those are just
CDs with a brush handle, there's a bit of wavering but they haven't fallen over yet

The cross fitting is very solid, if you're going with magnets I would stick with the cross too as it makes the joint much more solid.
As to positioning the stem, I just glued it dead centre of the oval base. As Chromedog says the cross piece fits in a specific place on the Valk anyway, and it seems to be pretty close if not dead on to the centre of gravity.