Yes it is safe it is just slower.
The really bad thing though has that you really have to commit. Many applications need a restart to work fast after you switch. Switching while running and the 650M might actually be slower if you started with the Intel GPU.
I suppose the frameworks need some reinitialization which only Apple in Safari really does.
Basically everything that uses some GPU framework or theoretically might use it will trigger the 650M (if you let Apple handle the switching). It will stay on until the App ist closed.
That naturally includes the Steam client because there is Flash in it. Spotify because it runs on AIR which is also Flash. Skype even if all you do is chat, because in theory you could start a video chat.
Pretty much anything that ever uses some sort of animation or video.
It doesn't matter than the Intel GPU is fast enough for handling a Musicplayer like spotify or all the other stuff.
gfxCardStatus even shows you which Apps would you'd need to close for Apple to switch to Intel on its own.
For me that is now
Pathfinder (which is really just like Finder and doesn't even have any cover flow, it is a pure non fancy text only finder with more features but none that requires even the smallest amount of GPU speed)
VLC (It is only not closed but currently not doing anything)
Chrome, Opera every Browser that can use Flash will always trigger the dGPU except for specially treated Safari
iPhoto (It works just fine on forced Intel if it is set already at launch, not happy with switching while it is running. Just try it. Used to work but slow now it just stop doing stuff. It doesn't crash or break anything.)
those I can all ignore and stay with Intel force on Intel but
external Display is in the list too and with Intel forced there is no external Display

Not that I would do anything the a MBA Intel GPU cannot handle)