I Up+B out of shield whenever I know it will punish (Rob Dsmash, Pika Downsmash, Meta Tornado, any of G&Ws aerials, edge get-up attack, Snake's first f-tilt, basiclly everything that is possible to punish) . I don't worry about Up+B diminishing because, for the most part, I'd rather deal as much damage as possible and worry about getting it undiminished later. As it stands, I think it's much harder racking damage without it than with it, and here's why:
Approaching with Marth can be a gamble, being that he can be punished easily from most of his better approaches. I think Marth gets the most damage in when his opponent is off the stage (juggling/edgeguarding), and Up+B puts your opponent in that position pretty easily. In the time that you are punishing your opponent while he is off the stage, usually you'll get in enough attacks to have an only slightly diminished Up+B (if you read them correctly), and the trouble of having to deal with the diminished Up+B is relieved to some extent.
Because Up+B is the most viable kill move in Marth's arsenal (IMHO), by the time your opponent is at 100%, the first thing you'll want to do is get it undiminished, and this can be done with grab jabbing/throws, fairs, and if you're lucky enough to get it, a dancing blade, in which all of these options will undiminish the Up+B quickly and easily, leading to a swift death should your opponent hit your shield with a punishable attack.
Major edit.
Edited again for clarification.