I have also heard that on the active leg i.e. the White waypoint rather than magenta, then the FMC will use the actual wind - can someone confirm if/when this is the case please?
From B757/767 FMC's, and there's no reason to doubt the same for B737, I was always told the actual cruise wind is used for the active leg and the next leg. i.e. the time & fuel data you see on Progress 1.
For descent winds I check the forecast and see if you will descend into a increasing head/tail wind. Because of the linear reduction from actual CRZ W/V to zero at rwy this effect will really throw VNAV PTH off its calculation. Perhaps then some help in Desc Fcst page is a good idea. If you are making a straight in from FL350 there's not too much chance to adjust, and a head wind at CRZ may turn into a tail on the way down and calm at Rwy. All good to bear in mind. If you are going to make an unknown ATC safari, with a 180 degree track change to finals, it's probably not worth it.
Best computer on the a/c is between the ears.