I've done it myself, firstly because I hate rolled posters, they are nightmare to deal with. But I wouldn't fold anything worth more than $10. Just crap that will probably never sell anyway and I can't bring myself to throw away. Stuff I use for gift wrap.
You can fold quite nicely with a long ruler, sliding between the fold to the edge and making a straight firm fold, then running along the outside over the fold to sharpen the fold.
Looks pretty much machine folded if you take your time. If I had a folding machine I'd probably get everything folded as I don't have much rolled of any real value and I just hate them. Except small posters of course, half sheets, inserts, Belgians, I don't mind flat.
As to value I think it depends on the poster/era. And as Chris says modern glossy stuff doesn't look as good folded and hand folded even worse. Whoever decided posters should be sent out rolled though deserves a slap IMHO.