[Tristan Lall]

Supply and cost issues factor into this as well. If you're in the market for relatively inexpensive screws, and located in Canada, Robertson parts and tools are easy to find (in wood and machine screws). If you're not, you're stuck with the thoroughly inferior Phillips.¹

If you're willing to spend a bit more, there are always Allen socket head cap screws. If you've got lots of money, you can even get Torx or Torx Plus cap screws. (Or maybe Posidriv, if you don't mind confusion.)

Neither can ergonomics be ignored. Allen and Torx (all varieties) need only 60° of clear space to rotate the tool (and you can get ball-end drivers for more flexibility in driver angle). Robertson and Phillips need 90° of rotation and perpendicular engagement. Slots need 180°.

Then again, only Robertson drivers are tapered so that the screw can be held without magnetic tips (e.g. on some Phillips tools) or flexible rings (on some Allen and Torx tools).

Speaking of tips, I swear Phillips exists as part of a shadowy conspiracy to sell screwdrivers. Have you ever noticed that they're the only screwdrivers that actually wear out in normal use? Whose bright idea was that?

The material surely matters. You wouldn't usually want an aluminum Allen socket, because it would tend to strip more easily than the common hardened alloy steel fasteners. Robertson tends to be the most resistant to stripping—even in materials like brass—because of the deep, precise engagement and more acute angles. (But avoid the combination Robertson-slot heads like the plague. They are frequently malformed and strip easily.)

As for thread size, standardize when possible (and combine that with drive standardization). The real questions are metric vs. unified national and fine vs. coarse.

It turns out that it's easier to find coarse threads in metric, so if you go that route, the choice is pretty much made for you. If you use UN threads, the fine (UNF) and coarse (UNC) ones are both relatively common.

Some teams, like the TechnoKats, have apparently gotten with the metric program and standardized on M5 as the most common fastener. (They just happen to be almost identical to a #10-32 UNC, which is convenient.) Most teams rely upon suppliers who don't stock metric hardware, and can't afford to invest in inventory of their own—so they use various bolts sold in Imperial units.

And by the way, check out aluminum pop rivets. They're better than screws in a lot of cases—and considerably lighter.

¹ Like the rest of this post, this is something of a generalization. You can find Robertson stuff in the U.S. if you look hard enough, or pay enough money.