It may be necessary for the 5:1:1 mix if the bark gets very dry, but it shouldn't ever be actually necessary for the gritty mix. The reason is, no matter how dry the soil gets, the Turface always absorbs water immediately. Once wet, the Turface allows water to diffuse into the bark, breaking its hydrophobia, so the next time you water (hopefully before the bark dries down again), the bark absorbs water more readily.

This is one of the reasons it's almost impossible to over-water the gritty mix unless you really work at it. Water flows quickly through, but the Turface absorbs plenty of water & then 'shares' it with the bark, so you end up with a nicely damp or moist medium instead of a soggy one. That physical property in conjunction with the large particle size work together to eliminate PWTs in a well-made gritty mix.

Al

Thanks Al!