It doesnt really matter to begin with. I however germinate in my medium straight away with only a little presoaking of the seed for a few hours depending on the medium. If its soil or coco for example i plant the seed straight into wet soil/coco without presoaking the seed. If im using rockwool then i presoak the seed for around 6hours.
Its once it gets into your medium that you need to worry about pH's.
If you are in soil try and stay between 5.8-6.5 with optimum being 6.2
If you are in hydro try and stay between 5.2-6.2 with the optimum being 5.5-5.8
If you are soiless try and stay around 6.0
J