Oh, I wouldn't rely on that at all :)

One example immediately jumps to mind that is deadly poisonous - Galerina autumnalis or the Deadly Galerina. This mushroom is most frequently mistaken for Flammulina velutipes. They don't look a ton alike, but if circumstances conspire against you they can start to meet in the middle. Volk recommends a spore print for every single F.velutipe one harvests just to protect against possible mistakes.

The only general rule of thumb one should take seriously with fungi and edibility is that there are no sufficiently reliable general rules of thumb.