Actually if you open the .osu file itself when you haven't gotten around to setting a preview time, it says -1 ms but I think it's around 40% of the song? So, technically speaking if you set it at 0 ms then it should be a valid preview time, I imagine some songs would go well with a 0 ms preview time, most probably don't but it's certainly valid sounding to me.
Even if in most cases 0 ms would be a bad preview point, I'm sure some songs would work out with it. I don't think AIMod should warn for that to be honest.