I do not believe you should only go by the pistils.
There are many ways to check a plant for ripeness.
Yes you can judge it by the Pistels But they should begin to receed back into the bud.
The other way is the swell and overall look of the bud. WHen a bud is ripe it will take on a different look to that of a flowering bud.
Then you should also check the Trichomes.
If you use these ways to check your plants for ripeness you wont go far wrong. BUt to rely solely on one method means you could be cutting too soon.
J