As I explained, yes you can but only for a limited time. For instance xcode 5 is only compatible OS X 10.8 and upward so it is likely that next year in june the new xcode will drop compatibility on OS X 10.8. Moreover to target new devices, new iOS versions you must use the latest xcode version thus at one point you'll need to make an update.
I also don't want to scary you either because it is doable, this is what I do since 2010 and I use it as my main system (I have a mac mini and a macbook pro but I prefer a big machine for my developments) and chances are that if your system works well with a given version of OS X it will likely work well with the next version. But it will always take some time to check the forums to resolve a particular issue or to check what people have experienced before making an update and breaking anything big. What I want to say is it's not straighforward because you are somehow always forced to update if you use it for developing apps.
I also don't want to scary you either because it is doable, this is what I do since 2010 and I use it as my main system (I have a mac mini and a macbook pro but I prefer a big machine for my developments) and chances are that if your system works well with a given version of OS X it will likely work well with the next version. But it will always take some time to check the forums to resolve a particular issue or to check what people have experienced before making an update and breaking anything big. What I want to say is it's not straighforward because you are somehow always forced to update if you use it for developing apps.