Just tried laying it flat and it seemed to work just as well. I did find that more material snuck out around the edges and I evntually had to open 'er up and put things in their place.
Protip (That works for me)
This may have been posted before but in my hunt for tips and tricks I did not find this particular one. I was getting tired of cleaning this thing. I found myself constantly having to pick out pieces out of the little window and clean the grooves with a toothbrush...