I know many do remove the autumn leaves before they drop. I do not. For the obvious reason that the tree needs all the back flow of nutrients it can get before going into dormancy. It is a natural process, where the decaying leafs turns off their photosynthesis as days gets shorter and colder. Nutrients flow back and are stored for the following spring in stem, trunk and roots. There the stored nutrients will change their balance of frost protection for variations in temperatures during the cold period, and when spring arrives they will be activated and be the first gasoline for a fresh start of a new seasons growth.
Removing leafs before time, is therefore a little too hasty in the wish of getting trees nice and tidy for the winter, taking away the last feed for wintertime.
I clean of the withered leaves from the pot surfaces and benches when they are dropped, too avoid pests and fungus. But not before. Only trees that do not drop there leaves when they are entirely brown and dead, are cut off, for the same reasons. Beech for example keeps most of their leaves on during winter, but that is not necessary when overwintered as bonsai in a protected place, away from snow and cold dusty winds.
Always make an autumn clean up, to secure the health of the trees, but in time – not before time.