Breathing - Does the wine need air?
As a general rule, I consider the practice of giving wine time to "breathe" before it's served to be somewhat overrated.
The idea behind it is simple: Wines that need aging may be shy on aroma and flavor when they're first opened, a characteristic that's sometimes described in winespeak as "closed" or "tight." Give them a little exposure to air, the theory goes, and you're providing a rough-and-ready substitute for the more gentle oxidation that occurs with fine wines as they age in the cellar.
While there's some truth in this, it's worth remembering that it only applies to certain wines. Most wines are fresh and fruity and ready to go as soon as they're put in the bottle, and letting these wines breathe risks missing out on their first blush of freshness. Worse still is the risk you take in breathing an older wine that's fully mature, as some older wines - like some older people - become fragile with age and may give up their spirit very quickly after the cork is pulled.
So I recommend breathing only for young, tannic wines, typically reds (or, even more so, youthful Vintage Ports), as a way to ease the initial "closed" quality or harshness from tannins. But if you do it at all, don't simply pull the cork, which exposes only a tiny circle of wine the size of a dime in the bottleneck to the air. Rather, pour a glass, and do it briskly so the wine gets a good exposure to the atmosphere. Then leave it for an hour or two, and you may find that the wine "opens up" before dinner.
Another approach, of course, is simply to open the wine at the time you serve it, take it as it comes, but if you find it shy, harsh and astringent, push back your glass and enjoy it after dinner, when it's had time to breathe.