Some journals request you to nominate colleagues to review your manuscript. If you know that a colleague is reviewing your manuscript, should you avoid meeting him for fear of biasing his judgement
some journal offer for reviewer to be non-anonymous if they like. If is critic constructive, such "open" approach can be fruitful. In such a case, direct debate is effective in order to improve your manuscript.
The answer depends on how limited a field you are working in, sometimes the field is so small that you cannot help doing so. Check the guidelines for the journal, if they don't outlaw it then there is nothing wrong with having normal academic contact with your reviewer.