There may be a "rule" for the width, but if there is I'm not aware of it. Since custom is custom, measuring the glasses will satisfy those glasses, and not take into consideration of them changing glasses.
Some glasses trickle into the sizes of goblets, and with them there needs to be more space. I can't say what the "average" wine glass measures, but I work backward to figure the spacing to fit the cabinet.
I start with the glass, and usually allow 1" for the stem between the runners the base slides on. The width of those runners, and the stock width to hold them will determine how close they are to eachother when hanging. Then for each far end, I figure a half runner.
What you might do is get an idea from the client what style they will generally use, so you can get into the ballpark. A tip for the runners, if you have several to do, just make up 8' lengths (or whatever your lumber is, and cut to length as needed.
Just a tidbit of information. Back when smoking was allowed in bars, one of the drawbacks to inverted glasses was that they would accumulate a haze from the warm air/smoke rising and look dirty. Even without the smoke, they don't seem to stay clean for very long.