The trouble with the warped tang isn't gravity. It means your tangs weren't ground even to start with or you got the steel too hot when tapering the tang. One other thing is that your foil and pull the tang when it's cooling. Try not to make your pouch too small. Leave plenty of room around the blade.
So, try to keep the tang as even as possible when tapering it and don't get it too hot. If the steel changes color, it will probably warp.
But, you can easily straighten a warped tang during the temper. Just clamp the blade to a flat piece of steel with the warped end facing out. Then temper the blade as you normally would. It will get rid of 95% of the warpage easily. I use 3 clamps. One at the end of the tang, one just past the beginning of the taper and one at the tip of the blade. Let me also mention that I only taper the tang befor the heat treat, I grind the bevels after HT. Hope that helps.
-John