Personally, whilst I always speak quite properly, I'll adopt some of the regional dropped letters and (to a lesser degree) vowel pronunciations if I'm at home and speaking to someone whom I might otherwise alienate by seeming too posh. (I'll also avoid using words like 'whilst' and 'whom', correct as they are.)
I think Zürich is about right, and that the idea that people are put off by 'posh' accents because they suggest you're 'nerdy' is wrong.
In particular, though, you won't be perceived well if the 'posh' accent is not your natural accent, and you've just adopted it, particularly if you adopted it for its prestige. A genuine upper class person speaking as such is not pretentious. Someone from another background who is imitating an upper class person usually is, and will be seen as, pretentious.
Likewise, if you're known to be from an upper class background, and try to imitate a lower class accent, you're at risk of ending up sounding like a total dick (George Osborne does a good job of this). I think the real point is that, if you seem like you're a genuine person, the way you speak is much less likely to be an issue.