I'd say only retake the LSAT if you have a good reason to think you'll do better. An exceptionally bad day with serious illness or something similar can be a good reason. It may wishful thinking to assume that just re-taking it while studying the same way you did before would help any.
I couldn't afford an expensive prep class. Apparently it's helped people, but my one friend actually got a 166 on her diagnostic and was hovering between 161-163 after the end of the testmasters course. Another girl we know that she took it with went from 148 to 154. A solid improvement, but hardly miraculous results. Plenty of other people get ginormous results, I read. But the trick to figuring out your milage isn't to average the two, but to think why you didn't do well to begin with (skills, practice, whatever) and what kind of learner you are.
My friends got a 179, 176, 175, 172, a 166, and a 158. The 158 and the 179 used TestMasters, while the middle people used LRB/LRG, I think. Many different reactions to the same set of stimuli. If you have killer recs and an appropriate explanatory addenda (perhaps in this case you'd have to talk about how test scores failed to predict your performance?) you might as well apply this cycle too.