I send all my stuff finally to VRML. In VRML a value of 1 = a meter. So I set the major grid to 1 and the snap grid to .1.
Everything then is built at these settings unless i need to combine stuff or adjust things ... then I will alter the snapgrid to smaller values. Or if working with a terrain ... then I may take the major grid to 10 or more and the snapgrid to 1.
No idea what scales are used in the various games though.
griff