I'm in the market for a new set of wheels. I've been noticing a style which I think is called "Deep V". I plan on adding gears to my currently single-speed bike (not a fixed gear). However, searching online, the majority (if not all) are advertised as being for fixed gear bikes. Are deep V wheels intended for fixed gear bikes? Is there any reason I would want to avoid them for a normal geared bike?
Deep-sectioned rims -- particularly Velocity Deep Vs -- became popular among fixed gear riders because they came in a variety of colors (bling) and the larger section made that bling all the more prominent.
However, deep-sectioned rims have many applications beyond urban fixed gear bikes. They're typically heavier and don't do well in cross winds, but in straight-ahead road riding, they are thought to be more aerodynamic.
As long as the rims come with machined sidewalls, they'll work fine for your purposes. The machining greatly improves braking by making the contact point for the brake pads more consistent.
All that said, if you're riding around town or even doing more recreational road riding, I think you'll be better served by a smaller sectioned rim like the Velocity Aerohead.
I can't help but elaborate on some of the previous excellent answers:
As indicated in the comment above by psycling, a deep section wheel has increased lateral strength; that is, side-to-side. Depending on spoke count and pattern, the built wheel will also have a reduced vertical compliance; it's less likely to absorb head on/up-down impact as well as torque applied to the drive wheel by the rider.
Lateral stiffness is apparent to me when grinding along on a tall gear, throwing the bike side to side, as typical with a fixed gear (or singlespeed). An average wheelset makes this movement feel 'spongy,' like every time you stomp on a pedal with the bike leaned over the front wheel is springing sideways a little bit. This flex is reduced with a deep section wheel, resulting in a more solid feel. I don't know about the detail of the physics on this, but my perception of riding a laterally stiff wheelset when I'm limited to one gear ratio is that less of my effort is 'wasted'. More fun.
The weight added by the rim around the circumference of the wheel is a factor to pay attention to; it will result in a cycle which feels noticeably slower to accelerate, depending on the weight cost vs. another wheelset. Some folks don't think this matters, but I'm not particularly perceptive and even I can tell the difference a few hundred grams makes here.
These characteristics are valuable when you're a velodrome track star, as acceleration from a stop isn't as important as cruising efficiency and there aren't any potholes. There's no strong argument why you shouldn't use a deep-v wheel on a geared bike, but if you examine the way you ride a bike with multiple ratios (fewer forehead-vein-popping climbs) you may find that it makes more sense to use a lighter rim with a more conventional cross section. I have ridden the Velocity aero rim as previously mentioned and it made a very nice, durable wheel, a good compromise.
Of course, there's also the style points to consider...