What do you think about allowing members to "manage tags" on the threads that others created? May actually help to make the tagging feature useful, since some users are bound to ignore tagging on their own threads when tags would make the discoverability of those threads useful.
Don't use 1.5 yet. So don't know if this exist in this version.
In 1.4.10 I use the tag add on. This have an option to force users to tag new threads.
At the beginning I am not sure if I get good tags with this, because our members are not used to it.
But after a while I can say users are not so stupid at all.
So for our forum it works.
Back to your question.
Just try it. If it don't work in the way you expect, disable it again. Any forum and his audience is different. An advice from others, not related to your forum, is only a guess.
The really impact you can see only by yourself.