The importance of finding potential mentor before applying for a postdoc
It is in general a good idea, not just for your chances but to ensure you will find a productive or at least welcoming group to work with.

The impact on your success chance I think depends on the country of choice: in some countries (e.g. Italy), postdoc calls are advertised as a public competition with a committee that should judge your CV impartially. 
In other countries, a professor may hire a postdoc on his personal funds and choose the candidate of his own will. In the latter case having at least some prior contact may be strictly necessary.