Obviously, as far as certain subjects go, this is perfectly acceptable - safety matters, for example. But even there, we have the old adage about children needing to burn their own hands before learning not to touch a hot stove. Sometimes you must give people the opportunity to form their own opinions.
Therefore, I would caution those of you who "know" about a given subject to be wary of exerting too great an influence over those with limited experience. Just because you don't enjoy something doesn't mean your buddy won't. After all, palates are different. To be even blunter, your palate may suck - you just might not know it - and personal taste is no reason to deprive those you care about from having an orgasm in their mouth.
But if you tell someone, "Oh, you don't want THAT," or, "Well, I'm buying this for my girlfriend, but I only drink Cab, so show me your Cabs" will basically ensure your night ends early - you had to have a big, ponderous red that she'll only have a glass of.
Perhaps you should do yourself the favor of opening your palate up to the suggestions of folks in the profession. After all, such individuals spend their lives drinking new things both professionally and personally and are constantly being exposed to some wickedly-tasting things that you either haven't heard of or are afraid to ask about because you're afraid of mispronouncing a region or grape varietal. Most of us booze-types are eager to share our discoveries, and are totally willing to teach you how to pronounce Viognier (vee-oh-nee-ay) and explain that it will pair well with lobster and sex.
And really, isn't that more than enough argument to keep an open mind?