As has been pointed out in other threads, the contact area of a ball bearing ball on a flat surface is practically zero. Since pressure per square inch is based on the amount of surface in contact, a load of 25 lbs on a surface area of 0 square inches is infinite psi! Well, I am exaggerating, but you get the point
Balls rolling on a non hardened surface will "roll their own raceway" for a while. Unfortunately, this process introduces extra slack in your rail system after a while.
This is why you should use hardened shafting. You could also search in the larger size steel warehouses for induction hardened chrome plated shafting. This is a prehardened commercial product, and is not excessively expensive.
If I were building a machine, I would not use recirulating ball bearings if I was going to go cheap on the rail rods. Its a case of not spending enough money to do the job right and is a waste. Go for a plain bronze bushing sliding on a chrome plated steel rail. For this, you can use the cheaper CPO "chrome plated only" shafting and forget the induction hardened requirement. This provides very low "sticktion", eliminates the problem of sawdust getting into a rolling ball rail, and is cheap, and will support a very high load without any difficulty.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)