"Yes, you should buy whole set instead of one or two bottles for maximum result. Whitening require complete care from A to Z." from counter girls or beauty advisors.
Not necessary, i can say. The main concern is whitening ingredients and concentration upon purchase. Cleanser & toner could be an optional, due to their limited concentrations. Plus, cleanser only stay on skin for 1,2 minutes, there is no time for whitening ingredients to penetrate skin as quickly as you imagine. Unless the whitening cleanser formulated by foaming agents suitable for your skin, then you could consider it. If your budget is limited, just go for serum/essence for it is often highest in concentration among the whole range.
Of course, you have to concern about skin conditions and whitening effect, if acne prone skin or oily skin wish to whiten their skin, just apply whitening essence in their daily skincare regimen. If dry, mature skin or pigmented, perhaps, besides whitening essence, nourishing face moisturizer is perfect to enhance the whitening effectiveness.
Tips: if you want to improve the whitening effect, you could apply skincare with fruit acids content. Fruit acids not only exfoliate dead skin cell, but also enhance penetration of whitening compounds, breakdown melanin to lighten skin significantly. However, fruit acid concentration, frequency of treatment is based on your skin types, dry skin which have lower water content of skin cells could be more frequent for their skin produce more dead skin cell than oily skin. Last but not least, don't forget about proper sun protection to reduce the melanin formation.