Will a glass bed warp as it is heated? I purchased a glass bed to use with my still-in-transit Ender 3. Since the bed came in before the printer, I pulled it out and used a flat edge ruler to see how flat the glass surface is. 

It appears the glass is slightly "dished" in the center from one side and "raised" in the center on the other side. I am wondering if this is a sign of a defective glass, or if it is likely that the glass may change shape slightly as it is heated?

If it does change shape, should it be put with the dish side up or down? 

Logic would say that if it does change shape, the bottom would probably get hotter since it is against the heater while the top is slightly cooler, so in theory the bottom may expand more "pulling" the center down?

I guess the bottom line is, should I send the glass back as defective or wait until the printer arrives and see what happens when I heat it up?
TL;DR
Yes, glass warps when hot. Use a physical touch sensor and calibrate it out, or swap glass if it's &quot;bad&quot;.

The further you go into mechanical studies like 3d printing, mills, and lathes, you will find out that nothing is perfectly flat. Everything has a tolerance to it, whether the manufacturer provided it or not. Better manufacturers provide the tolerance data.
As you have or will find out, yes, glass does warp when heated. That is because you are not warming the glass evenly. When hot and cold spots form in glass, the glass tends to warp up or down, depending how the heat travels.
It's common for 0.01 mm warp when a glass bed is brought to printing temps (60-100 °C). If one has a larger format bed, like my Vulcanus at 500x500, 0.03 mm changes are possible.
One can remediate this warping, as it usually appears in the same place during the heat cycle. So if you calibrate after the bed is heated, you should reduce said errors. Another more comprehensive way to fix this, is with a physical touch sensor and create a grid map of the bed geometry.