It depends why you are growing it. If grown for decoration leave the flowers on if for culinary use then keep the flowers cut off and the foliage young as this improves the flavour.
nope that means your too late. the buds are what you cut, not the flowers
You should deadhead your yarrow to make it flower more. Cutting it early will make the plant shorter and give it more flowers. The plant will look more attractive because when it is taller, it tends to flop.
Chrysanthemums flowers will die and should be cut off to encourage more flowers to bloom.
It grows slow in height, but makes a bunch of babies that you can use for making many plants so you have enough for cooking or tea - you dig the hole plant up and will discover that all the mini-plants - you then loosen them from the motherplantroots and can then plant them with good distans. remenber to remove the flowers so they will not make bad babies, and when picking for cooking: be sure not to cut to low or you will have a bare spot on your thyme.
flowers... that have been cut...
yes, if they are from an acid loving plant, ala philodendrome.
Yes, flowers should be cut from basil because once basil flower, they will start to die. This is selena22110 saying, yes it should be cutted if you want to harvest it still, if not you can leve it for beauty.
cut.
depends on plant type. Flowers of sulphur stops disease
Subject to opinion but I think a female would like the flowers more and they are more traditional, for a male a plant might be better depending on their age, personality etc.
Yes, Casuarina does produce flowers but they are small. Casuarina is an evergreen shrub so it's not a typical plant you'd think to cut flowers from.