I always said I would rather shave than have a balding head. That is what I did when I first noticed thinning, and others thought I still had a full head of hair. Less than 5 years later, I have almost no hair on top. So it is your choice if or when to shave, there is no "best" time. If you want to be proactive about it, shave now. Or if you just want to try it out, it will grow back just like you have it now. Or you can wait.
One thing that I think worked for me was deciding before hand what I would do and when. Having already made my decision, when the thinning started, I just did it and did not go through the mental adjustment that a lot of guys do when they a struggling and trying to keep thinning hair.