I would highly recommend using a banana grooved shaft with the factory iron rockers. Some reasons are mentioned above. Yellow Rose mentioned the rocker grabbing or sometimes called friction welding or galling. See last two pictures below of a damaged stock hydraulic rocker shaft without banana grooves from using the stock 273 rockers below. The higher the spring loads you have on your engine the more likely and quickly this will happen. As mentioned above a dedicated race car with high idle and limited run time at low RPM will extend the time this takes if at all.
I would suggest as the first choice in shafts for iron rocker arms is to use the factory 273 mechanical rocker arm shaft as there are other differences will point out. First the banana groove which was already discussed.
Second is a there is a second oil hole drilled for the push rod cup which provides pressurized oil directly to the cup to lubricate and cool another critical friction point.
Third the two oil holes in the mechanical rocker 273 shafts are smaller that the one larger hole in the stock hydraulic or aftermarket one hole shaft.
Fourth if I recall correctly is the mechanical 273 shafts have the bottom oil hole and the banana groove clocked slightly towards the valve springs rather than straight down to optimize the oiling where the load is when the rocker is in the lift cycle. Something like the minute hand on a clock when it is at 6:35.
Some aftermarket rockers have an internal groove around the inside to feed pressurized oil to the pushrod cup oiling hole in the rocker arm - Hughes for example and the Crane iron rocker arms. The stock 273 rockers do not have this groove. Chrysler added the additional hole is in the shaft instead.
Will the pushrod cups get oil without the groove in the rocker arm or added oiling hole in the shaft. Yes it will get some but it will not be as effective as a second hole in the shaft.
So for me my preference when I use mechanical rocker arms, regardless of application, street or race, is to always use banana grooves shafts and a shaft with an oiling hole for the pushrod. If the rocker has an internal oil groove in the rocker body to get oil to the pushrod cup then I may consider shafts that have a banana groove and no additional oil hole in the shaft for the pushrod but it will depend on the application. I tend to like the additional insurance of the additional pushrod oiling hole in the shaft.
I also added a couple shots some 273 mechanical rocker shaft to show the two oiling holes and the oiling hole sizes.
