Interesting topic....VNAV is only any good if you fly the entire LNAV track.....9 times out of 10 this isn't the case (unless you've correctly anticipated what ATC wil give you, which can work on a familiar route but be prepared to change quickly) and therefore FLCH from FL100 in conjunction with the 3 times rule seems to produce reasonably good CDA's. Of course ATC sometimes throw a spanner in the works but this method generally works for me.
Back to the initial question.....I can only echo the above in that garbage in = garbage out. The more info the FMC has the better VNAV works.
2W2R
