From a guy who has done a lot of heat treating of steel: Those advising you to leave 10 thousandths over desired dimensions are spot on. Normalizing the steel BEFORE you begin machining it helps reduce inconsistencies which can result in stresses after the hardening phase of the heat-treat.
After normalizing, annealing, and machining, coat your block with a coating which reduces scale and oxidation.
Now for the super big thing: Heat needs to be equal on the part. If one side is hotter than the other at quench time, it will increase the chances of distortion/twisting markedly.
Assuming you are using a metal that has a liquid quench (1095 (water/brine quenching), O-1 (pronounced OH-One, oil quenching), ect) having your quench liquid already warmed up can help, and then you want the cooling from the quench to be EQUAL on all sides. You don't just want to toss it in a bucket, you will find "swishing it" to keep bubbles from forming on it (while quenching) helps to ensure all sides are equally cooled in the quench. Bubbles act as insulation, and thus you have less effective cooling where the bubble is at. Also, some metals (such as 154CM) benefit from a post sub-zero quench (we would sub zero quench 154CM between plates of dry ice while in a chest, the metal would squeal).
The temper: each alloy of steel has a different tempering curve, know your metal, and know your curve. KNOW what is the target hardness you are shooting for. I would look at advertisements from major brands on their 123 blocks, and see what hardness they brag about, use their good research. Some steels temper at low temperatures which can be achieved in a home kitchen oven. But, you will need to measure the actual temperature of your home oven. It is not uncommon for home ovens to be off by as much as 50 plus degrees. Pre-heat your oven, let it stabilize for at least one hour before you put your metal in it for the tempering phase.
