Gerry,
It's not necessary but just a suggestion from most of the tremec dealers, as the assembly lube and any foreign debris is washed off by the ATF and forms a messy slurry. Does it lubricate any better/worse than straight Syncromesh? I don't know. It's similar to running a new engine for the first time. It sure can't hurt. But I can't say you'll notice any benefit either
.
Mike