I used to slide down on the old Qantas not quite flat seats. It doesn't seem to happen on the A380s.
Someone will probably pop up now and tell me the angle hasn't changed.

I recently took a Lufthansa biz from KL to FRA and it was not quite flat but contoured so perfectly that I didn't slide down.

As stated above, the nose is usually higher than the tail but even so, I used to slide down over time on the old Qantas seats.