Probably a stupid question, I don't know much about this stuff. Is there any reason not to just use a 2.5A even if you're not connecting power-hungry USB devices? If it doesn't cost significantly more and it doesn't hurt to use 2.5 anyway, it's something I'd rather not have to worry about.

Feel free. It'll only draw as much current as it needs. The rating on the power supply is just an upper bound of what it can (theoretically) provide.

That makes sense though I don't see a 2.5A adapter to order separately on their website (because I might be needing one later).

I have several of these, they are 2.5A and good quality.

http://www.amazon.com/CanaKit-Raspberry-Supply-Adapter-Charg...

Wouldn't the more common use-case for power-hungry usb devices be to have a powered hub to drive them? Then you don't have to worry about browning-out the RPi itself, though of course the cable clutter is worse.

We will be sourcing the 2.5A power supply but it really is only necessary for very power hungry setups!

Would two webcams quality for that?

That would be pushing it for powering through just the RPi. Webcams are typically rated for 500mA each and the RPi has been running around 700mA usage. So with the recommended increase, I'd wager their power usage may have gone up to 900+mA.