It is said that spores give off a gas that inhibits further flushes. I have never encountered this. I have seen spores so thick on the surface of the casing soil that they crust over and cause the next flush some problems - of course, if you are growing any sort of bulk the spores can be harmful to those with sensative lungs or asmatics, the spores can coat the walls, fall upon your food and ruin carpet - they also tend to discolor neighboring fruit. Golden teachers, when the caps are entirely clean dry an authentic gold color that is quite beautiful, these are all good reasons to pick just after the veil breaks. I hear that pre-veil breaking mushrooms are more potent and I tend to believe it but I don't think they are noticeably so. I can verify that aborts (when picked before they rot of course) are, gram for gram, much more potent but I really don't know why.
There is something about a fully open cap however, that is estheticaly much more pleasing than picking before the veil breaks. Finally, because fresh mushrooms give one a different and more majestic effect (again, I don't know why, it could be something as simple as a psychosematic effect), pick and eat them with no concern for their maturity, simply select the ones you want to consume, pick them and stuff them in your mouth - there is no better way to do this. Those who wait to properly dry them and then make tea, or crush them, or dip them in honey or whathave you are missing the most valuable aspect of growing your own. I have friends who swear by munching them directly from their growing place from top to bottom without picking them at all - chomp chomp chomp.