Do I have to soak seeds before sowing?
No, not at all ;-)
The long period that the seeds are outside for will provide plenty of moist and cold stratification.
The seeds will absorb moisture soon after being sown into the dampened soil. Rain and melting snows will drip into the flat lid through the ventilation slits and keep the seeds sufficiently moist--so no pre-soaking is ever neccesary.