depends on your ticketprices and if you win a lot or not.
I'd say expand there where you risk to run out of places if your team keeps developping as it does.
e.g.
say you have 1000 / 200 / 50 / 5
and you started out averaging with 500 / 150 / 20 / 1
but in the meantime you steadily grew to 900 / 200 / 30 / 2
then you should expand the 200, ofcourse, but also the 1000, since you are getting close to filling it up too. For the other 2 types you have some reserve.
You could also start altering the ticketprices, which might trow the balance in an other direction... and just expand every type.
If you go take a look at some of the biggest teams in BB you will find out some prefer high prices and smaller arena's, while others lower their tickets a lot and expand much.
You'll notice the tactics are very widely spread appart, so I guess you'll have to try to find your own way at what you think works best for you.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.