so you are tying to drop the output of a 4 cell lithium ion battery to not fry a typical ham radio ?
depending on the diode and the current the forward voltage will change, I have some that are about 0.2V...
best way to find out about a diode is to download the data sheet on it
this may or may not be the kind of diode you are looking for
www.diodes.com/_files/datasheets/ds28007.pdfbut you can go look at the forward voltage VS current graph to see what I mean about the forward voltage changing
on the DC-DC LM-series converter
a switching one I assume ?
I run the LM2596 with battery voltage on the output and they seem just fine with it, (I have never looked at the data sheet to see if it should be ok or not)
but they do take some current that way, so you might want the diode there just to save your battery power.
if you are running an LM318, or any of that type, use the diode or make sure you properly set up the chip or you will just be getting lucky if the chip does not fry.
hope I answered your question,
but can do better if I know all of what is going on.