The first set of leaves on the very bottom usually do dry up. Those TWO should ony be turning. As for the other leaves, if you notice drying on the ends, I clip JUST the dried part of the leaf. You need those fan leaves! They supply the sugar to the buds and you don't want to clip any off unless it's dire straits. I have nut burned my plants before and they responded well after I ajusted the ppm. Flowering has not been a source of stress for my plants unless done too early. You are at week five and I would flower if I were you. Stress causes are heat and over fertilizing. I blow a fan on my babies to thicken their stalk up a bit. Remember that your plant usually doubles in size after you begin flowering. If your bottom leaves are the only ones effective, you may not be burnning them. It may just be the sprout leaf and they die off anyway. These babies are pretty durable.