And related but important enough to warrant a new post IMO - cog quality for running fix gear:
If you ride the hills, run velodrome quality chainring (if possible, often it's not) chain (easy) and cogs (well, they do cost more). I strongly recommend running 1/8" for everything. Reason? Downhill, RPMs can approach the ridiculous. I've done high 40s MPH in gears like 42-17. (For brain dead easy math, 100 RPM in that gear is 20 MPH. So, 40 is 200 RPM, 50 is 250 RPM.) Pedaling smoothly at those RPMS? Yeah, right! If your chain can possibly come off at the loosest part of the pedal cycle, sooner or later, it will. And the consequences will be excessively exciting. That's a given. Might end your ride, your rear tire, your frame's paint, some spokes. some frame integrity, some intact skin, maybe the integrity of a bone or two.
Two ways to prevent this from happening. 1) run a tighter chain. (This could be expensive. With less than velodrdome quality cranksets, the wobble in the chairing circle - not an issue at all with gears - may mean the chain goes full tight momentarily at one point of the several pedal revolutions chain cycle. This will act to put forces on the BB and hub bearings they were never intended to see. Or 2) put the proper slack in the chain and use running gear that is as resistant to derailling as possible. (Basically you are trying to go as far from a modern derailleur chain and cogs as you can get. Those nice rounded teeth that run so quiet (and shift so nicely on cassettes? Bad!)
I run: EuroAsian 1/8" cogs. ~$35 last time I looked. Expensive, yes. Quality steel and they last a long time. Square cut teeth and noisy. Popular at the velodrome for the racers without deep pockets. Isuzu Eco 1/8" chains. $25-30 depending on color. Quality the same for all colors and very high. Again, square and noisy. Also velodrome popular among those on a budget. (And the guys riding the good stuff don't mind seeing the Isuzu and EA on the bikes of the wallet strapped because they know it's good good and they won';t behind a chain throw crash.) My chainrings are all 1/8" but from a variety of sources. For my 110 BCD bikes the Mojo rings are good, round and OK for wear. Haro BMX rings are excellent but I haven't seen one in many years. That bike of my photo has a sweet Sugino 75 crankset with rings as good as they come (and that crankset is so round, setting chain tension is a joy!)
The cheaper you go with your crankset, the more important having chain-throw resistant chain and cogs becomes. Also the more attention you have to pay to setting chain slack right. I have a couple of Surley cogs with a nice gold finish and rounded, quiet teeth. Scares the **** out of me on my bikes with 110 BCD road cranksets. I'll only use them with that sweet Sugino 75 where I can set the chain minimum slack at a proper 1/2" and it never goes beyond 5/8" (I feel much more comfortable with the EA cogs and gold doesn't fit the color scheme so those Surley will last my remaining lifetime plus!)
The money spent on the Isuzu and EU is money well spent if you keep riding fix gear because they wear so well. I do track chain stretch and replace when needed but I keep them. The day may come when all or enough of my cogs have worn past being good fits with new chains and I should be able to start a "new life" with both for most of the miles it took to get past the original "life". That original has been well over 20,000 miles with several replacements of my most used 16 and 17 tooth cogs. Another 10-15,000 miles should have me covered a long time! (This personal aging issue being a friend here.)