It is normal for plants in flower to yellow their leaves and die off slowly (not all of them of course). This is simply the buds using the nutrients in the leaves to get bigger. However the way in which they are yellowing is important to note. If the yellowing is spotted, and there is also tip curling and/or burning, then you have deficiencies or nute burn, which this season has taught me are the most complicated and frustrating problems a grower can come across (besides pythium aka "root-rot", that one is the worst - & exhibits similar symptoms to the former).

Post photos here if you think something abnormal is occurring.