Well, since that's what the manufacturer recommends, it must be the right approach. 
Makes sense, too. While foam may cosmeticly fill the gap, it does nothing to evenly support the tank: in areas where the foam is compressed very little, it is not providing any support at all to the tank.
Foam is good for obviating any point stresses on the tank that may be caused by nailheads, etc. For tanks that only need support on their ends, I guess is works. For those, maybe it helps with racked stands, too.
Your 125, though, is pretty long with respect to its height. Supporting it just on its end may put too much shear stress on the sides, thus the manufacturer's instructions to fill the gaps with hard, noncompressible material.
Makes sense, too. While foam may cosmeticly fill the gap, it does nothing to evenly support the tank: in areas where the foam is compressed very little, it is not providing any support at all to the tank.
Foam is good for obviating any point stresses on the tank that may be caused by nailheads, etc. For tanks that only need support on their ends, I guess is works. For those, maybe it helps with racked stands, too.
Your 125, though, is pretty long with respect to its height. Supporting it just on its end may put too much shear stress on the sides, thus the manufacturer's instructions to fill the gaps with hard, noncompressible material.
