Ph, I have found that they vary from flush to flush, now that could actually be a temperature variation between flushes but I don't think so.
Unless you have a way of testing alkaloid levels, you dont really know if that variation was you, or the shroom. Are you growing with cloned tissue or starting with spores? If you start out with multi-spore, you will see variation in potency because you have geneticly different shrooms on one sub.
If you are using a clone, there should be no variation regaurdless of flush.
I have seen variation in mushroom shape, and simple things like that can affect your trip. A good example would be portabellas. They are 3rd and 4th flush button mushrooms. I've noticed mushrooms get bigger in later flushes but a shroom isn't going to change it's chemistry as it ages. In fact as the substrate becomes depleted of nutrients, if any thing they should lose potency being that certain amino acids are required precursors for the alkaloids.