Whether or not you should use the boards, and when, is entirely up to you
I like your thought about why so many rapid-cyclers are here - it probably is at least partially due to us going in and out often!
I so far have only been on the board while in a hypomania and then went pretty much straight into a depression - I haven't really been baseline since I joined.
My rapid cycles are usually because I'll have probably about 3 bouts of depression in a year? With at least one, shorter, hypomania. I haven't done a good job at tracking my ups, so I don't really know for sure. But my episodes tend to last a few weeks, although I can have depressions that last many months.
I believe that I will continue to use this board lots even when I am baseline - I find psychology interesting, and I use a lot of the other boards as well and not just the bipolar board.
I also like knowing that I can be around here to help other people - I might be feel normal, but other people on the boards who are new, or whom I've befriend, well, they might not be and they could benefit from my prescence.
As to when I am baseline - well, I haven't been baseline since I got diagnosed, but I knew I had bipolar for years and years. When I'm baseline I don't put too much thought into my bipolar - but I do keep a regular routine and when I'm baseline is when I'm doing that the most consistently. I'm also better at avoiding triggers when I'm baseline, and I am better at self-care. I know that I'll eventually have another round of depression or another round of being overly active, but I try to not worry about and I just accept it'll show back up again whenever it feels like it.