Bboostr is well worth its money for anyone running more than just a few tumblr blogs. That is for a variety of reasons, but the most important is its loop feature that allows you to set blogs so that the queue never runs out.
It’s just one little click in your queue settings in BBoostr, after which you can also click the shuffle button and then you will never have to worry about the queue running out again. That saves a lot of time and stress and allows you to focus on manually reblogging posts on more blogs because you have freed up the time you would otherwise have to allocate to checking the queues every now and then. I recommend to have a queue that is as varied as possible for this, at least ten different blogs and ideally more.