I travel regularly with Easyjet, on average !'d say once a week. Most of my travel is alone and with hardly any luggage, so I usually purchase the cheapest ticket possible with no options such as an allocated seat. I'd say, based on my N=1 experience, in the past 100+ flights, there is no strategy to minimise my chance for a middle seat. On my routes, I usually have a 80-85% chance landing an aisle or window seat. Late or early check in does not seem to make a difference. The algorithm seems random to me.