There isn't any real clear cut answer.
Like you I have been using mailwasher to bounce back spam. Since most of the spam addresses are forged I suppose it doesn't really matter whether you delete or delete and bounce.
Using the bounce and delete option with one email address I have gone (over a 2 month period) from 10 spam emails a day to 1 a week. But with another email address I've gone from 2 a day to 10 a day.
Where is the logic?