With Dead Money you're not going to miss much if any map markers since Dead Money is fairly on the rails story and location wise.
I know I didn't finish the DM map, but it's too late now. I never used the Service Routes in any of my play throughs, so I'm pretty sure I missed a marker or two. Did anyone else not use Service Routes in Dead Money? A lot of the video tutorials show the player HAVING to use Service Routes to get around, but I never used ONE. So, I'm worried I may have missed out on some good loot. :cryvaultboy: