Yes you do need a toner.
Toners help skin return to its natural ph level. PH is the balance between acidity & alkalinity. Skin is at its healthiest when its slightly acidic. Acid prohibits bacterial growth that can cause acne. Alkaline skin is more prone to wrinkles & sun damage. A toner also assists penetration of treatments and moisturizers. If you are interested in seeing results from your serums and moisturizers, tone before using them.
Another benefit of using a toner is to get the hard water residue off of the skin. Chlorine and hard minerals are left on your skin after you rinse off the cleanser products. These minerals can dry your skin & cause wrinkles.