Some of my happiest times are when I'm at -3 stab. Being rock bottom you can only improve and whatever events you get can't really hurt you too much. You get free policy clicks, and if anyone declares war, you get a free stab level.
I laugh when I'm at -3 already and get dirty stability hit events.
My ideal stab is +2 cause I hate it when I get a random or historical event that gives me a stab boost, and I can't take advantage of it.
It is hard though when you are a country with heaps of different religions and you can never keep your stab up. Playing as the Ottomans, I am very rarely +3 at any satge in the GC, so am happy if its positive.