For thyme, it's after. Oregano blossoms are as deliciously aromatic and flavorful as the leaves, so pick them when the mood strikes you. For parsley, the time to pick is before cool autumn weather toughens the leaves. Mints are at their best before flowering, so pick then. Harvest oregano and thyme when the flowers are barely open.
Rosemary leaves are easy to dry.
Pick them whenever you want. Pick sage flowers when the top blossoms are barely open. Wait another month or so to snip leaves. Pick tarragon when it's lush, before cool nights cause it to yellow. Basil should be picked before the plant sets flowers.