Well it is not forbiden as far as I am concearned, most of these followback are done in the serviece section of the forum here is quick link to it: https://bitcointalk.org/index.php?board=52.0
followback let's say is good strategy to gain followers faster but when you get to some point of followers that you feel comfortable with I'd say that you stop to do follow backs as followers should naturaly come and without followback you might be seen as more legit. Other thing is that there are a lot of ways to buy twitter followers - which I don't recommend as most of the time you will buy bots that will follow you as mostly people have "farm" of some sort full of bots which will enable them to sell their serviece. With followback you can get "fake accounts" aka bots as well but in my opinion this should be on lower proportions.