Adding sulfites to your wine. When?

This is a topic that you will get a diverse level of responses.  A lot depends on what type of wine you are making and the style you are trying to create.  Most red wines are made with sulfites added at the crush and at bottling, at a minimum.  If you are barrel aging your wine, you should maintain a safe level of sulfite to prevent oxidation and problems from harmful bacteria forming.  Here is a clip from the Wine Spectator website about the topic of sulfites, and when to add them:

"Malolactic bacteria are extremely sensitive to sulfites, so even modest quantities hinder them. However, oxidation due to low sulfite levels isn't a problem at this stage because malolactic bacteria generate a blanket of carbon dioxide that essentially seals off the wine from oxygen.
And many wines are better without malolactic fermentation. Some vintners, such as Lingenfelder, want the crisper style that comes from preserving malic acid. After primary fermentation is complete he adds enough sulfite (about 70 to 80 mg/l) to inhibit the malolactic bacteria in his Riesling.
Barrel-aging, which gradually exposes wine to air, is part of the program for many of the world's most reputable wines, including Bordeaux, Napa Cabernet, and red and white Burgundy. So vintners constantly monitor and tweak free-sulfite levels to ensure that oxygen exposure remains controlled, rather than excessive. "Every time we rack, top off or mess with the barrels, we analyze SO2 and add enough to increase it to 20 ppm [parts per million], maybe more," says Steve Test, winemaker at Merryvale Vineyards in St. Helena.
Brawny Cabernets tend to spend about two years in barrel. When it comes time to bottle, vintners usually want about 25 to 40 ppm of free sulfite. That's enough to slow oxidation and inhibit unwanted microbial activity, such as Brettanomyces, a spoilage yeast, which can impart pronounced leather and barnyard character.
At bottling, different wines require sulfites for different reasons. Reds, due to their tannin and pigmentation compounds (anthocyanins), are more resistant to oxidation than most whites; but reds are more susceptible to microbes because they usually have less acidity. Late-harvest wines demand the highest levels, typically more than 50 mg/l free sulfites (sugar tends to bind with sulfites, and vintners need an ample sulfite supply to shut down microbes, which might otherwise wreak havoc gorging on the residual sugar).
Whether introduced during fermentation, barrel-aging or bottling, sulfur is an integral part of the winemaking process at the vast majority of wineries. Although its power and utility in stabilizing wines is unmatched, a few hardy producers embrace a noninterventionist philosophy that frowns on sulfite additions.
Amity Vineyards in Yamhill, Ore., offers an assortment of organic wines, one of which is also made without added sulfites. Called Eco-Wine, it's a Pinot Noir with a youthful, fruity style.
"Asthmatics are some of our most ardent customers. They'll buy 15 cases," says Myron Redford, owner and winemaker at Amity. "But they're a very small number. There is also a huge number of people who think they're allergic to sulfites, who blame sulfites for any reaction to wine."
Redford started the Eco-Wine label in 1990, at which time sulfite additions were not permitted for organic certification (current regulations allow a maximum of 100 mg/l added sulphites, with no more than 35 mg/l of free sulfites permitted at bottling).
Winemaking without sulfites can be a zero-sum game. Redford must take special care when handling the Eco-Wine: it can't withstand any time in barrel without oxidizing, and in order to eliminate potential microbial problems, he performs a tight filtration, which can reduce richness and complexity.
The rare cases in which individuals suffer allergic reactions to sulfites notwithstanding, most vintners agree that eliminating sulfite additions is both unwise and unnecessary. "There's a tendency to polarize these issues, to make it black and white," says Ramey. "But it's not; it's a question of being judicious. Let's not throw out the baby with the bath water."
- Wine Spectator, Feb. 2003, "Inside Wine: Sulfites"