Addition: introducing oxygen into the unfermented wine ("must") at the start of fermentation, to encourage aerobic fermentation, is a good idea. This is because yeast need oxygen to reproduce. Since very little yeast is actually added to the must by the winemaker (at least at home =) the little yeasties need to spruce up their population before they can go to work effectively.

Once the population is up and fermentation is on its way, introducing oxygen into the fermenting mix is a bad idea.