FWIW, the only reason to refrain from downshifting into first is because you will wear out the synchro for first gear very quickly. It is the heaviest gear in the gearbox, and requires the most braking effect to get into gear. When you downshift into first, you are asking the synchro to stop the heaviest gear in the gearbox, so it will wear the most. However, if you understand gearboxes and rpm, it is possible to do this in a way that minimizes the wear. Unfortunately, since you are a noob to manual shifting, I wouldn't recommend it until you develop a greater experience base from which to decide how to use your gearbox.
Yes, I have rebuilt gearboxes and I can show you pictures of the results of downshifting into first when you don't do it right.