Senesce or senescence is the biological process of aging, abscission and eventually death. Flowers usually have the shortest period of longevity amongst all plant parts. Flowers senescence can be gradual or quick.
Senescence brings us the question of deadheading. Is it important to deadhead? The answer is no. For eg., rhododendrons don't need to be deadheaded after even flower goes through senescence and death. Deadheading doesn't affect the natural function of the plant.
However, deadheading of spent flowers does make the plants more aesthetic and in some cases pushes for more flower production. For eg. in pansies, if you pick the withering flowers before they completely die, it pushes the plant to keep producing more flowers. The plant's ultimate goal after all is reproduction and seed production. If flowers are pinched off in some species, the plant puts more energy towards making more flowers to try everything it can for seed production.
In some cases, it is possible to halt or extend the natural senescence and allow the flower to keep going for a while. Good examples are cut flowers, like roses and carnations that you can buy with plant food. Once added to water, the plant food allows the flowers to push senescence and keep flowers fresh for extended time. But eventually senescence occurs and they wither.
I recommend deadheading for Rhododendrons, Azaleas, Peonies, Daylilies, Rudbeckias, Shasta Daisies, all for aesthetic reasons. Deadheading is especially good to deadhead Lavender stalks to keep a round form of the plant. Also, there are ample uses for dried lavender! However, if you want to let the flower set seed, then leave it alone and let nature take its course. As refreshing as new growth is, so is senescence to me. It is a reminder of the natural life cycle and seed production in plants.
Included are pictures of a Southern Magnolia showing senescence. Look how gorgeous the bronze beige petal color is. Also, notice the inner seed stalk that is getting the signal to grow full swing, now that the flower has ceased its growth.
As you add new plants, think about the period it takes for the flowers to get to senescence, how long do they hold their shape, will they need to be deadheaded to keep up with aesthetics, and finally could you use them as cut flowers indoors and prolong their senescence to get most of the flower? If you want prolonged senescence in a flower, then I would say a hybrid tea rose is a winner. Like the yellow tea hybrid rose pictured below (a new addition to London Town's gardens) will last for a really long time in your yard as well as in your vase!