It's possible that the shadow mask might not be attached strongly enough on a standard TV to allow it to be mounted horizontally (i.e. cocktail style) for an extended period, but I don't think it would be damaged just from angling it back 10-15o or so.
A lot of people are using angled screens, and an awful lot of those are TVs, so if it was a drastic prob, I think we'd have heard about it by now...