Sub 4 sub is one of those gray areas, where overall my quick answer would be don't do it. But for my first 3 months, it's probably one of the factors that helped me get engagement in my channels. But you have to be very strategic about it, and only sub back to someone whose actually commenting and interacting with your channel. If it's someone whose clearly just copy and pasting stuff, then that would be an automatic delete. But if it was someone who I could tell by their comments actually watched my videos, I would give their channel a chance and see if I could build some relationship. However, if you're going to take this route, have the mentality from the start that this is only very short term growth and you want to get away from it as soon as possible.