In both cases I think you want to say: I wouldn't be here unless it was important. I wouldn't come by unless it was important. or I wouldn't be here if it wasn't important. I wouldn't come by if it wasn't important.

You simply need to change "wasn't" for "was" in both sentences to achieve your desired meaning, but both the phrases are perfectly formed. Tom

The sentences are both ok, but is that what you wanted to say? The way it is written, it means that if it was important, you wouldn't come by.