|Hey baby, you goin' my way?|
My rationale? I don't care enough not to tell them. They'll figure it out on their own (at least, as far as the armor class), and they could always tell how much health the thing they're beating up has based on how hurt it seems to be (it's limping on the ground, its thousands of tentacles dragging on the stone floor beneath it/it roars in defiance of the wound you've inflicted on its underside and rears up for another attack/the blood gushes from its struck eye like a fountain of gore, and it lashes around in fresh rage/whatever), and I'm not interested in secrets, really.
I haven't found it impacts their enjoyment in the least. In fact, it even let me heighten the mystery when I do pull the certain forwards, and say, "you don't know how much health it has." Why not? I hit it, right? I can tell how much I hurt it, right? Is it healing itself? Does it not feel pain? Is it a "boss fight?" Is he not what he seems? What's the matter with it?
Plus, did I mention that I was lazy like that? Any chance I get to fob off the mental math to the players and spare my puny intellect the horrors of having to... to... crunch numbers on the fly (!) is a change I'll take any day. Half of the time I'll generate random treasure hoardes in front of my players, and let them roll the dice for it. I won't tell them what it's for until after they're done, but they get to roll the dice and be part of the random-generation fun. (They:"Alright, sweet, I got a 20! That's some awesome treasure, right?" Me: "No, that was the wandering monster table. 20 goblins burst from the door to your north, bristling with spears and swords and looking directly at you...")
Your mileage may vary, but being open about everything is just my style.