The point to raising the mast is to lower the level of hydraulic fluid in the tank. This is to prevent the tank from siphoning when the suction filter is removed. (Big yellow spin on). This is not necessary when changing the return filter.

So really do you HAVE to raise the mast all the way? No, but it will probably make a bigger mess. (But good luck not making a mess when changing it anyways because of where its located in the truck.)