IANAL and I am in the US, but I'll play devil's advocate and present the worse case scenario. Your app launches, Big Bad Company comes along and says you infrin

IANAL and I am in the US, but I'll play devil's advocate and present the worse case scenario. Your app launches, Big Bad Company comes along and says you infringed on their patent or trademark and sues you. Without the legal structure in place there is a chance you will be held personally liable. I think YC makes that statement for people who have no live product and are just a team either with an idea or idea in development. What they are trying to avoid is having to make costly changes to your filings once you join YC. Personally, I would say that if you are that far along, preparing to launch, and plan to make a business out of your app, then file. Spending money now can potentially save hundreds down the road. But again, IANAL.