Round bar under pipe shoe

We do that here also (which I have not seen before) but for bare pipe resting on structural steel. I was told the reason is that the bar presents a reduced area to the pipe that 'could' trap water and therefore reduces the chance of corrosion. I don't believe though we put bars under pipe shoes. Corrosion of piping is a potential problem, I'm not sure I'd be that concerned with corrosion under a pipe shoe.

However, the standards on your job will govern what you do.

Conceptually shoe supports are provided to increase degree of freedom for the pipelines in the longitudinal direction.
Particulary in steam lines, being velocities very high, the chances of pipe movement and thermal expansion are more. shoe is a sliding joint and it will act as a stress reducer in the pipelines. As there is only a line of contact at shoe and round bar joint it is obviously good. But care has to be taken to properly support the pipe rigidly for specified lengths elsewhere. With line contact chances of corrosion will be less when compared to area contact.