There is also the altrnative to bombing which is to stay on edge and make very wide radius turns. In this case you will almost be bombing but will traverse the slope in a way that you will be moving downwards at a far greater rate than you are moving sidewards. The advantage being that you remain on edge while getting comfortable with speed. This way you can be gradual between slowly traversing and bombing.
A big +1 here... Not sure why an instructor would tell somebody to flatbase and bomb a run to "get used to speed" unless you were just on the bunny hill and top speed in that case would be a quick jog.
Otherwise, it's way better to stay on an edge. I think it's sloppy boarding to flatbase. Do very light carves but you're pretty much bombing. I do this on all long traverses, it's actually faster than flatbasing, and you rest each set of muscles every few seconds or so (instead of getting calf pump or quad pump by the end of the traverse).
Also, not sure what the hype is about keeping your weight centred. You actually need to have your weight in all different areas depending on the snow, your speed, etc. Best way to feel out what weight does is to start off heelsliding, then gradually put more weight on your front foot, then back to centre, then towards your back foot. If done correctly you should turn more downhill with weight on your front foot, then hold that position with it centred, then turned back across the hill with weight on the back foot. Do the same exercise on your toe side to feel the same feelings.