Well the point of the thread is for asking people's opinions.
I say, if it's an important matter, you need to be honest, especially if they're going to find out about it anyway. Being honest about something is much less likely to ruin a relationship than keeping quiet and letting them eventually find out on their own.
If it's something minor, don't flat-out lie about it, but sometimes it doesn't hurt to hold back from being completely honest about it :P usually in the cases of opinions.
Generally the question "Does this make me look fat" translates to something to the effect of "Tell me I look good in this" (unless they actually look very bad in it in which case tread carefully.)