If the freq is an even number of 12.5 khz steps from the megahertz integer, it will go in. If it is an odd step, then you will need a freq step of 7.5 khz or 2.5 khz to get it. As Grog indicated, some radios have the 6.25 khz in them for UHF and some have the 2.5 khz for VHF. Since there seems to be no 6.25 khz Project 25 standard as of yet, you don't need the ultra-narrow freq steps.
Grog,
The Icom/Kenwood NXDN digital narrow band is interesting since it does meet the 6.25 khz narrow band stats. From what I have read, any FCC license is for the center of the assigned frequency. So you won't have any dibbs on the extra spectrum you vacate if you do put up a ultra-narrowband (6.25 khz) system? I would like to have the first NXDN system up in SC, but to clear interference out of Greenwood County, I would want to be on 453.10625 mhz or 453.09375 mhz. I understand that the NXDN Icom repeaters will work either 12.5 khz analog or 6.25 khz digital like the APCO 25 units do, but if one goes to a ultra narrow channel, you can't do that. I wonder if you would be required to re-coordinate to go to it.
The Phase 2 Project 25 is going with a 12.5 khz channel with data riding on half and digital voice on the other half if I understand correctly. This meets the 4800 kps data rate per channel standard. That doesn't meet the standard for walkie to walkie (for interop) and is why the ultra-narrow band UHF was put on hold.
Bob