Of course when I hear the question "is it safe", all I can think about is a dentist drilling. But in this case I'm guessing most of our listeners are too young to remember that movie so we will move on from there and just point out that it probably is not safe or a good idea to run a beta operating system on your phone.
Because your phone is something you use daily you don't wanted it behaving poorly or acting strangely. Beta operating systems are not released for good reason because they're not ready to be released yet and they really are meant for people developing applications for that phone.
Sure you might want to get the latest features for the iPhone but the fact is the operative system will be released when it's ready.
Wait until the public release happens unless you really need to be running a beta version. Believe me unless you're developing applications it's likely you will regret putting the beta version on your phone and it's very difficult to go back to release version once you do.
Wait for the release version that our advise.