I have friends that use the 5 x 5 ones without issues, none of them use the steel footers. There is a friend that uses the steel footers under a 4 x 4 Kallax unit. I personally use the 4 x 4 unit but no steel footers.
If I did buy a 5 x 5 units, I would personally just put it straight on the floor like I have done with the 4 x 4 units.