There is a footnote in the rules now suggesting that you estimate the line when you both can agree.
3.(A) 10.1 HILLS: In addition to having visible Crest Lines beneath some terrain, newer boards are much better at depicting where Crest Lines actually are by using gaps in the terrain. In many areas of these boards, most players will be able to agree on where the Crest Lines actually are underneath the other terrain of grain, brush, woods, or buildings even without visible Crest Lines. When players can so agree, we encourage them to use the actual Crest Lines to determine LOS.
Due to this footnote, I have gotten into the habit of asking my opponents before every game how they would like to handle it.
So far my experience is that the picture is divided: Some want to play like it "always was" (i.e. wood depictions on the "lower" part of the hex are considered to rise from the "upper" portion of it, which is in accordance with the current rules as given), some want to use the crestlines in the proposed manner of the abovequoted note only where they are printed on newer boards (which so far only applied to grain IIRC), some want to use guesswork for crestlines, but only where gaps in the artwork makes the guessing easier, and some at last want to use the guesswork approach for all boards (even the oldest ones, where determination might be difficult).
IMHO, the best strategic approach for MMP would be to publish all new boards with printed crestlines. This would create the basis of institutionalizing the approach so far encouraged by the footnote. Then eventually, at least for the boards with printed crestlines, the rules should be changed that their use is mandatory.
If there ever is a Map Pack v2, then the maps might be redone with printed crestlines. In case this is combined with a general rules change, this would create the problem of how to play all the old scenarios. As it would be rather easy to ignore the crestlines and to have a look at the publishing date of older scenarios, it would not be too hard to overcome.
For VASL it could comparably easily be implemented to blend in or out printed crestlines (however, a prerequitisite would be the availability of "official" boards that include printed crestlines.
von Marwitz