with poor quality media, the area most likely to cause prob lems is the edge of the disc. if the dye is uneven on this part of the disc, you get a failed disc if you try to burn all the way out to the edge.
setting the target size slightly lower than the full capacity of the disc compensates for this.
if you are confident that you are using quality media then by all means burn out as far as you'd like to. if you start getting weird burn failures though, it should be your first troubleshooting step to set the target size to the lower value.
hope that helps.