I saw in a documentary call "L'erba proibita" a men, Franco Casalone, who wrote 2 books about growing weed ("Il Canapaio" and "Il canapaio 2") telling that if you wait a little bit more and you get your weed without pistils it is stronger.
Here it is what he wrote in his books about the picking of the flowers (traslated a little by google and a little by me):
Some cultivators prefer to collect soon, when the greater part of the flowers is still supply of pistils and the plant is in the moment of its maximum upgrades them riproduttive (if it came impollinata). The total psicoattive effect is lighter, but nearly exclusively cerebral. Others collect latest possible, when there will be the greater resin amount, and its effect stronger and more will be extended also to the rest of the body. To collect (and to try) some female flowers every week, for a period of some weeks until the final harvest, will give to the cultivator a collection of samples of flowers to all the maturation stages and will create one base in order to decide when to collect in the successive season. [fig. 17, 18,19,20]
If you know italian you can get it here:
and here the other one,
Actually Franco Corleone is at address arrest becouse they have found 400 grams of charras in his house. I hope he will be free as soon as possible, like weed.