You're lucky to have such soft water that the pH can drop to 5.0!
I've had discus go through pH crashes completely unscathed while other fish, e.g. Corydoras, were burnt. However, doing water changes is always a good idea with fry. Start with a 30% - 50% change just to make sure you don't make the parents overly anxious. Then increase it daily.
A common mistake is to assume fry do not contaminate water because their total mass is so much less than adult fish. In fact, water quality declines rapidly from ammonia accumulation. You'll notice that fry breath very rapidly and every breath releases urea from the gills. Urea breaks down into ammonia in the water, then nitrite, then nitrate. So water quality drops quickly in tanks with large spawns and large water changes are highly recommended. I'm always concerned when people post that they can breed discus in small (~20 gal) tanks. Yes you can, but larger breeding tanks favor better fry development.