It's something I've always used when 1 op is driving both desks, as it seems so logical ("SouND Q.3, Go" is less of a mouthful than "SFX Q.3, Go" for the DSM) - no chance of confusing with "LX", either visually or aurally. If using separate ops I just write "Q.n" in their scripts (& LX & SND in mine). For broadcast it was, & presumably still is, "Grams". I always try to have the cue numbers & details on the same page as the script - eyes up & down is enough, without adding side-to-side, but more importantly, if you drop the folder & the pages scatter you won't be crawling round in the dark hunting for the page with the next cue written on it.