I'm putting up a 6ft vinyl privacy fence. The holes reach 42 inches (city mandates 36 inches). I was planning to put a couple inches of gravel in the bottom of the hole before filling with cement (per fence kit instructions), but I'm getting a bit confused now I'm thinking about it. The thing is, I've seen my holes in the clay soil hold water for days, so gravel at the bottom of the hole would just trap water with nowhere for it to go. Now, it's below the frost line so it shouldn't heave the post up, and it's a vinyl post so it shouldn't rot, but it's not doing any good either, so what's the point?
The clay, as you state, will trap water with nowhere to go as well. By digging the whole deeper and lining the bottom with gravel, what you do is provide a place for extra water to drain to until it can dissipate/drain.
Without that extra space for water to drain, it can fill up the hole above the frost line and cause lateral displacement.