Make sure you get aluminium tooling plate because that will be guaranteed flat. Also, the glass needs to be float glass because that also is flat (horticultural glass my be rolled glass and isn't flat). Furthermore, don't be tempted to have the glass toughened - it may be safer but the toughening process can make the glass bow (I know from experience).
If you build it right, with 3 point levelling you'll have no need for any sort of bed compensation. I've got to the stage where I can just power up the machine, select an object and hit print. No pre - print checks or setting up required (well maybe a quick squirt of 3DLac). The start gcode heats the bed, homes the axes, heats the hot end away we go.
HTH
Ian