> Maybe just casually say "and I'm going to use :tab which allows me to select multiple lines and line up the equal signs" or something similar.

No, I wouldn't want them to casually say anything... that's why the subtitles would be an unobtrusive compromise, since you can just turn them off. I think this would be even more important if the screencast was targeted at new users of the language or framework or whatever, since the viewer might be confused and wonder if ":tab" was something to do with the syntax of the language.