When a call builder builds a call, we try to build the sweet spot on the top of the call. Myself and Brian, and a few others, mostly use a wood soundboard in our calls. We pick the tightest grain in the soundboard and put that on the top, and when you surface the call, you scratch it up going with the grain in the soundboard, not across it. That way when you play the call you playing it across the grain in the wood soundboard, which gives you the best sound. The grain in the soundboard should run the same way as the grain in the call does, to get the best sound.
With a glass, slate, or other material used for soundboards, it's just a crap shoot, most builders will surface the top by how they sign the back of the call, or with a picture call you surface it where it won't distroy the pic on the soundboard. With a glass call you don't have to search all over the call for the sweet spot, like you do on a slate call. A slate is porous, so you have to find the best sounding spot, a glass isn't so it doesn't matter where you play on it, it's uniformed so it sounds the same where ever you play.
If you buy a glass call that isn't surfaced and if it has a wood soundboard in it, look the soundboard over closely and decide where the grain is the tightest, and surface it with the grain in the soundboard, that should give you the best sounding spot on the call. If it isn't a wood soundboard then it really doesn't matter where you surface it.