In other words, insist on previewing all  photographs (or at least a subset) before judging to get an idea of the quality of material submitted.  This enables you to successfully tailor the range of your scoring to the actual range of quality.  This does not mean that you change your criteria for scoring, only the strictness of their application.  

Lack of any preview can lead to messy, unfortunate situations, especially with numerical scores.  If you do start judging blind, everything will depend on the first several slide/prints.  You may award them 6's and 7's (out of 10) thinking that they represent average photos in the group.  But if they are worse than most of the photos that follow, you will then be stuck at the high end giving out strings of 9's and 10's when the better photos appear.  The reverse can happen if the first photos are among the best in the group, the result being that you are eventually "trapped" at the bottom giving out 3's, 4's and 5's.