Older leaves often will turn yellow as the plant begins to use its stored sugers for bud
production and is completely normal.
If your plants are yellowing in the earlier stages of flowering(the first month) then its perfectly fine to spray them with fertilizer, just remember to dilute to a quarter of the regular strength.
The main reason they say don't spray is because it can cause bud mold, but as long as you dont go overboard with it i wouldnt worry about it.Just keep an eye on your humidity.