Brake rotors seem like a bad place to experiment. If Eric says .455 is accceptable then I'd take that as gospel. Here's my shade tree way of checking for obvious warp: after visually inspecting for grooves or scoring, I lay the rotor on a flat machined surface. (I used my Rockwell unisaw table before my cousin gave me a truly flat granite stone of the type a machinist would use). Bad warps will be obvious like the scoring, suble differences might be found with a feeler gauge.

Setting up a surface gauge while the rotor is still on the car might work except they are so sensitve that it would probably just lead to confusion.