Board Thread:Dark Souls/@comment-91.120.90.210-20140616222205/@comment-24682999-20140616231245

Optional bosses are any that you do not absolutely HAVE to fight in oreder to make it to the Throne of Want. This automatically means that the only bosses that are not optional are the pair of Dragonriders, Looking Glass Knight, Demon of Song, Velstadt, Guardian Dragon, Giant Lord, Throne Watcher & Defender, and Nashandra. This is because you need only reach the minimum soul memory requirement to open the Shrine of Winter and access Castle Drangleic.

Also you can choose, but you must defeat a boss or two to get to a Fragrant Branch of Yore to unpetrify Mirabeth. The easiest way is to beat the first Dragonrider, get the Priestess to let you into Undead Copse, then beat the Skeleton Lords. Get the FBoY from the chest in the cave you meet Gavlan in for the second time in Harvest Valley. (I believe there is one in that chest, correct me if I am wrong)