I think the primary factor that I've seen is whether a journal consists of mostly locked content. In that case I'd say it's offputting to subscribe to someone, but not give them access to see anything you write yourself, because then how can you ever turn that into a two-way relationship? However, if most of your stuff is open, subscribing without granting access isn't a big deal.

That's how I treat my own journal. I only access-lock content of a highly personal nature. Therefore I only grant access to people with whom I have a significant relationship, whom I feel I know and trust. I guess it could be considered similar to the distinction between "friends" and "acquaintances".