According to FCOM 2, FMC preflight, that entry is for forecast cruise wind and optimises FCM calculations.
On the CL old manuals ( big,white ones - 07/94 pg 19.50.10) it says :
Cruise wind line: Entry of forecast true wind a top-of-climb helps optimize the FMC computation. If no entry is made, the FMC assumes 000/000.
The specification top-of climb went lost on the new books,hence the confusion.
we'll let the Boeing guys learn about this, and hopefully we'll have the corrected version asap.
bgds Alex