last summer my cousin was over and almost had a fit that I let my basil flower, apparently according to her leaving those flowers on makes it bitter. Hope that helps!

You can extend the life of many herbs by pinching off the flowers before they go to seed. Basil is one of them. In general, flower and seed formation take a great deal of a plant's strength, and can diminish flavor. Some like to use some herb flowers in salads, however.

I notice the garden centers frequently cut down many herbs to make them branch out better. I have found, in my conditions (little sun) gentle tip pruning is better.

In any case, with herbs, the most convenient time to prune is when you need some of the herb, IMO. It is usually possible to harvest what you need in such as way as to enhance the desired shape and general health of the plant.

where: tips (usually 3-4 leaf sections) & cut back to just above a node

Like Eibren - part sun conditions need less snipping...
Heeding warnings about self sowing - I snipped lemon balm too much last year & lost some plants. This year I cut back only 3 plants to 18" high.

I leave a small section of oreganos & thymes unclipped because I like the flowers & know the good bugs do, too. I snip the flowers after a few days & the plant is nice & tidy again.

when: morning after dew has dried (ideal to spray plant off the day before) - inspect carefully & you won't have to wash them

how: scissors & a basket with fine mesh (can rinse right in the basket if herbs need rinsing

Parsley does not branch. The leaves grow from a basal rosette. Pull individual stems from the base to harvest. More will grow. If you cut the leaves from their stalks the stalks will just die anyway. If the plant puts up a central stem it is about to flower. You can delay this by constantly cutting the stem but the parsley will win in the end.