If you know you will never want a dk on the other accounts definitely go solo and as mentioned keep your main in the area for the group quests. If there is a possibility you may at some point want a dk on the other accounts you might as well do all 5 of them. 5 dk wreck pretty much everything in a global cool down and a few seconds for elites. With the changes to the xp rate in the old world and BC you can skip all collection quests and still have plenty of xp to get you to northrend, I'm actually looking at staying in outland to 71 so I can skip some of the many northrend chains that have collection quests, horrible collection quests even.