If you are getting packed down places in the bedding you might just need to take out some bedding if it looks like it has been converted to castings and put in some more shreded newspaper. How long has the bedding been in the bin?

Bedding can get packed down, when it does you need to just pull it up and break it apart a bit. You might be better off only doing this to a portion of the bin at a time so you aren't disturbing your whole population all at once. Paper bedding gets packed down quicker than cardboard but needing to fluff the bottom layers of the bin is not uncommon.

As to how often, that depends on lots of things. I would say as often as is needed. I usually dig a trench in one end or the other of my bin to put food in, if I notice that the lower levels are packed down then I know I should fluff a bit and add some fresh bedding before adding the food. Next time I might fluff the other end or middle.

I dumped out my mini-tubs into their 3rd month because I wanted to replace the burlap & like you, thought it was too dense but they were fine! I only fluffed the top when I fed them.
BUT now that I graduated my 2 mini-tubs to a 10-gallon plastic I HAVE fluffed up the bottom once (2nd week) because I wanted to harvest some of the BLACK GOLD but I had too many babies in there so gave up and just layered bedding, food, bedding. I do have 2-1" PVC with lots of holes running thru the long ways (I saw that in some of the make-your-own container sites) of the tub because it gets warm with the decomposing and I am in Hawaii so I don't want the worms to overheat and you know. . . croak.

I think when I do my 2nd tub (stack) I am going to use some kind of netting or stronger burlap, leave it long for "handles" so if everything gets too packed I can just shake everything, LIGHTLY, instead of dumping their home and scaring them!