For the Classic which I assume is the same in this regard as the NG: If you don't put any winds into the DES FORECAST page, then the FMC will use CRZ wind linearly interpolated to zero on the ground. However, if you modify the route during the descent the FMC will then use the current actual winds! So if you re-enter the top way point in the LEGS page, may find yourself getting above or below your profile. But if the actual winds change significantly again, the aircraft may have excess or not enough energy.
I have found that the best thing to do is just monitor the descent, see how it goes and make as few adjustments as possible taking into account sequence and traffic the subsequent impact on my profile!
Personally speaking and a point of personal interest on the Classic, I've not noticed much benefit of putting wind in the forecast page. One can reduce incidents of DRAG REQUIRED messages with fake tailwinds, but then the speed can also become irritatingly slow too. I prefer minimal input unless significant wind is forecast.