Just discovered that when dubbing in high-speed mode (which preserves user-set markers), that going to DVD-R does not preserve the frame accuracy of their placement. It looks at if they're reset to the GOP headers, and thus can be off by as much as 15 frames (~500 mS).
Initially, I thought I must have done something wrong, since I never noticed this before. On the first burn, I did a couple segments, then removed the DVD-R and later put it back for the rest, before finalizing. Since I still had the content on the HDD, I re-did it, doing the full disc burn and finalize in one pass, without removing the disc. The results were the same.
I then tried bouncing material from the HDD to a DVD-RAM, and the markers were still frame accurate, as they remained going back to HDD. Evidently, when reformatting from VRO to VOB for the different disc formats, marker accuracy is sacrificed.
Just a word to the wise in case you're contemplating marking specific points on a program, for slide shows, etc. They're fine for normal segment marking, but to make sure that skips are clean (no brief flash from a preceding segment), you might want to step a bit into the next segment before laying down a marker.