If you want to display it and the lines really bother you, back it. If you don't want to display it, lay it flat in a humidity free environment and it will look as good in 20 years as it does today. I would not roll a folded poster over a long period of time, it stresses the fold lines. I hate linenbacking posters, but if I do, and I have as recently as last week, it has to be for absolute necessity. The GWTW poster is nice, but it is not rare, thus not a necessity in my book.
T