I would do only the back. From aesthetic point of view. But I'd ask what the building code says..or what your contractor says. I know in the kitchens you don't have to wrap around the backsplash..bathrooms, I'm yet to find out myself:)

I think the stone can be grinded a bit to make for a thinner backsplash. We asked the same question and got a positive answer. But I must say it sounded a bit vague too. So I would just ask and see what your builders tell you.

Thank you aprilnevends! I met with the fabricator yesterday & he thought because of the size of the vanity (backsplash will be 5") the 3cm would look great. Also, the right side of the vanity is an inch wider than left side so if we do a sidesplash (?) it should look balanced, esp with the 8" backsplash tub surround. I will be able to decide at install if I want right side sidesplash after all.

I prefer end drawers on a vanity that long, making the side splash less critical and giving the person at that sink elbow room. So you do need a side splash to prevent water from going down the wall when the sink is close to a wall. Also, the counter edge may not be straight and tight to the wall, and a side splash covers the gap/uneven cut.

Side splashes made with the 3 cm counter material look very chunky. If 2 cm is available, I'd use that. Or I'd use an ogee edge profile to minimize the blockiness. Personally, I think all that Caesarstone in the adjacent tub and large vanity area may make it look too plain and I'd use tile on both areas to add interest.

Thank you for your feedback mayflowers (z8)!