It depends on what you'd prefer actually. If you were new into the phone itself and wanted to test out a few apps to see which one would fit you best, you should choose Just once, means the phone would open the file/perform the action with the app once without setting it as default. This is good if you want test the app and carry on with no long term effect on your phone. If you were sure what you wanted (for example, if I get a new phone and for the browser for example, I'd always put Chrome and choose Always when opening something, so Chrome becomes my default browser) then choose Always. This would make the app your default app for performing the action/opening the file. For example, if you found your "perfect" music player, you just choose Always the first time you open a music file and every time you open a music file from that point on, this app would be launched. Of course, this can always be changed from settings > Apps > App you've chosen as default > Clear Defaults. At least that's how I know it
Hope this helps.