Whether you pull it up, or down, is really up to you and your project needs.
As mentioned prior
If your code is looking to trigger on a falling edge or a
0, you would want to use a pull up to keep the level high when unused.
If your code is looking to trigger on a rising edge or a
1, you would want to use a pull down to keep the potential at 0, util your external source pulls it up.
Now, pulling resistors should have a high ohmic value, enough to overcome the noise you need to eliminate, but not so strong that your external soucre can not overcome it.
Generally, by pulling down you will conserve a few mA of power.
Lets say we have a 5k pull up @ 5v, that is
1mA per pin; when running off of batteries, every mA is precious.
When we pull down, we are generally just burning off the EMF noise, consuming a negligible number of mAs.