The reason why there is no longer a set or recommended placement is for a variety of reasons.
As already mentioned, to some extent it's to do with giving the young members choice. But we should also consider that for some families sewing or otherwise affixing badges might not be practical when they may need to remove them for a younger sibling to wear the same uniform later on. And some may wish to try and limit wear and tear so that they can recoup some of the cost of the uniform by reselling it afterwards.
I myself would prefer an official placement for "key" badges (promise, six/patrol, awards, annual challenge/adventure, and sixer/leader/second, local area emblem) and then allow choice for how interest badges are displayed. Brownie sashes without any order in my opinion look jumbled and scruffy, and oversized non official or fun badges look out of place on all uniform.
In my experience this is the sort of small thing (picking up girls on where they have placed badges) which tends to contribute to feelings of petty sentiment that turn girls and parents off Guiding.
Also a reminder that if you are a leader who is not happy with the fact that there is an official policy of no set placement or requirement to display on uniform, to email CHQ as well as posting on here. They do listen to some feedback from members on issues like this. After emailing about badges for TSS before I was informed that TSS wanted certificates not badges. Two years later they introduced octant badges because members said that they wanted otherwise. Other examples include: return to the use of "uniform" as an approved term and the legal team are now looking at being able to call TSS Rangers again.