First of all, how are you doing 2 preptests a week ON TOP of your TM HW? Is anyone else having trouble completing it all or is it just me?? Secondly, I was advised by my TM instructor to not touch a preptest until AT LEAST after the 7th lesson. That way you will have a pretty good grasp of the material. HTH.
I'd say you start giving preptests after the second diag. at TM. That way you would know some of the important concepts and techniques and would have a lot of actaul LSAT practice ques. under your belt. I am thinking of doing the same. Hope i rock the second diag.