"Most guidelines for broccoli say to fertilize up until the main head forms."

Hmmm, never heard that before in all my years of growing it and it raises all sorts of conflicting questions in my mind. Way too many variables involved to accept that statement at face value.

That said, just as with watering, fertilizing isn't something that fits any fixed "schedule". It is as the plant shows you it needs it. So without knowing your soil composition, soil preparation at planting time, current soil nutrient levels, fertilizer being used, watering regimen, etc. I can only make the general recommendation if the plant begins to yellow or shows a marked decline in production, feed it.

Dave