It depends what you're definition of safe is.
If you're that worried about being banned, don't modify your console. You take your chances when you use backups online - everyone does.
Exactly what I tell everyone that comes to me to get their console flashed, what I say goes something like, "Its safe right now, in this moment in time, assuming you are following correct procedure making and patching your backups and using updated firmware, but, as soon as M$ finds a way to detect your using a burned disc in their console, you will get banned as it violates their TOS, regardless of the law. This moment could be tomorrow, it could be next week, it could be next year, they may never catch you, only time will tell".
So in short, its up to you to chance it, when you do, if/when you get banned, it would be entirely your own fault, no one else but you.
To answer your question, as of this moment in time, its safe to play online, but, like I said above in my quote, you never know if or when M$ will find out a way to catch you out.