My perception is that making someone nosy on an issue is preferred to assigning it to them (allowing them to assign it to themselves if they think that is appropriate), unless the issue is of higher priority or someone actively working on the issue really needs the other person's input in order to move forward. But these are my own rules of thumb, and a discussion of how best to use this field may be appropriate.
I personally think issues should not get assigned unless the person it is being assigned to is known to accept such assignments, e.g. by having asked for them to happen.
For example, I would prefer not to be assigned any issues, because I'm unlikely to act on them in the six months - unless, as David says, I'm the *only* person who could move the issue forward in that time.