No the API is forward compatible so if you start an app today than all future Facebook updates will work with it until the particular update expires. Heres how it works.
You start an app today using Facebook 2.5
Facebook gaurantees that each version will be available for x amount of months I think around 16 months.
So you can use the 2.5 api until that 16 months expires.
Once that time expires, your app will be forced to use the newer API say version 30.4 or whatever.
At that point you will need to update any broken parts to the newer version 30.4 API or stop using Facebook.
Now thats your current app.
If you take the exact same copy of your game designed for version 2.5 and install it as a brand new Facebook game in a month or so, you will find that your required to use the latest version in this case version 2.6.
Many of your features will still work from the old app, in fact it may work just fine. Facebook only pushes small changes at a time. However you might find that a persmission had changed or the way you used to post a score has changed just enough to break your code. At which point you'll just need to update your code accorindingly and you will once again be able to use the app.
As for asking for a user defined string of permissions, the official plugin currently doesn't do that. There are third party options, for example I have a plugin that I use for Facebook in order to do some more extensive tasks. These options are not the official plugin though so they are not allowed in places the the Scirra arcade.