I'm guessing this is a typo and you meant positive (+) castor. I'm pretty sure any amount of negative caster would make for a, well, very interesting, driving experience.4 degrees negative castor is absolute minimum on anything I own. Close in on 6 degrees and it starts effecting steering effort noticeably, but you will never have death wobble there.
My Fords are always right around -4 degrees. Never had a castor issue with any of them. My Toyotas start at zero and I take them to -4. Dodges are often around -2 and I take them to -5.