IMO, you shouldn't be subscribing to other people in the hopes that they subscribe to you. From what I can tell, that trend of "sub for sub" doesn't really happen here on Tapastic. Maybe on DeviantArt or something, but here on Tapastic most people who have lots of subs have earned them by making a comic that's worth subscribing to, updating it regularly, and maintaining a presence in the comic community.
Plus, when you subscribe to a comic, you're going to get notifications through Tapastic itself as well as emails and the like, so god help whoever seriously subs to every single series on the website in the hopes of getting 1 or 2 subs in return lol
As a creator, I also rarely check the profiles of my subs. Unless it's someone whose username I recognize (so personal friends and/or family who made an account to subscribe or something like that) I just check the sub count and move on. So I'm definitely not spending my time sifting through my subs just to subscribe to their series because they subscribed to mine (especially because the ratio of readers to creators is a lot higher). That's time I could be spending working on my comic so that I can actually get subscribers (and for the people who are already subscribed) 