What I've seen is to wait until the entire cake is colonized, then wait 2 days, THEN birth your cakes. If you leave uncolonized parts of your cake exposed, they could get contaminated by the green stuff and ruin everything in your fruiting chamber. My understanding of mushrooms of all types is they will put out chemicals that will inhibit growth of other organisms so if there is nothing exposed to colonize, then your cake is safe to birth.