I too used to use real world weather, but have since had to go through and preset the weather at the normal routes I fly. From what I've seen, setting temperatures at or around freezing (Paris and Toronto @ 30°F), have resulted in the message for de-ice being prompted. However, when I go to Doha and Sydney (80°F), this message does not occur. So I'm thinking it's temperature based, but it definitely could be using time/seasons as well.