Ok, seems some of you need some info about mushrooms in general.
Once a cake is colonized with mycelium (the white stuff) it doesnt need anymore spores to make new flushes. The mycelium IS the mushroom(s). In fact mushrooms are made up of 100% mycelium. The mycelium just changes form, goes from vegetative form to fruiting (reproductive) form.
If some spores get deposited on a cake its fine, however if its a thick layer of spores it will hinder future flushes. This is because those spores germinate (sprout so to speak) and then compete with the mycelium in the cake. They basically fight over food, air and water. This makes your cake waste time and energy that it could be using to make mushrooms. It takes a THICK layer of spores to make a difference in most cases. So a little spore dust on your cakes isn't that big of a problem. In most cases you won't notice any diminished flushes. Most growers pick their mushrooms right before or right after the veil breaks. This is mostly to keep the fruiting chamber clean.
When you pick mushrooms in the wild, most of the time they have already started to drop spores. Those mushrooms are still fine to eat. Picking the mushrooms that haven't dropped spores yet in the wild is a big NO-NO! Its best to let wild mushrooms drop spores so that a new generation may grow and spread.
Hope this helps and clears some stuff up.