I've had four steel road bikes in my lifetime (Schwinn, Motobecane, Sekai and Gitane), and only one (the Gitane) had a flexing problem. It wanted to upshift when I climbed out of the saddle, which, as you might imagine, was inconvenient. Of course, that was in the days of non-indexed shifters, too, so I can't say the same thing would happen with modern shifters or not. None of the others steel frames were any problem at all.