You do need to know. For example, if you are sailing upwind on a triangle course, if you just sail to the apparent wind you can end up chasing it into a big speed building, bearing away curve that keeps the tell tales flowing, but isn't doing your VMG any good. Same thing downwind ( with a spin especially) , relating to the true wind v. your speed build is key for VMG, and deciding if it's a velocity header or if you are coming up too much. For Mary, the larger waves are usually moving at an angle to the wind, I look for the tiny parallel rippling , not the waves themselves. I think it's not one or the other on the true v. apparent, but sucessfully integrating the contrasting information of the eyes and sails that keeps you on top of things.