I have taken two old PTs as "diagnostic" tests myself, before really diving into the studying. I honestly can't tell you whether you should or shouldn't take one. For me, knowing what I could score on my own told me that I really did need to study early and study hard- but then, I'm a procrastinator by nature, so some extra motivation is a great thing. Because the LSAT is nothing like any other test, I feel good having taken a full one.There are lots and lots of opinions against that though- here's one LSAT tutor's arguments against diagnostic tests:
http://lsatblog.blogspot.com/2009/06/free-lsat-diagnostic-tests.html
One thing that is often recommended is to do some of the old individual games by category as you are learning the different kinds (Basic linear, etc). The LGB should be giving you some example games to do with each chapter. If you find that you are having a hard time understanding one type of game, the first appendix in the back of the LGB lists all of the old PTs and each game's category. So for example, if you were having a hard time with the basic linear games after completing the LGB section, you could also do PT 30 Game 4, PT 34 Game 3, PT 31 Game 3, and PT 34 Game 2.
So essentially, if you are working with the PowerScore book, I would suggest reading a chapter, making sure you understand it, and then supplementing that chapter with additional practice if you don't. Don't worry about timing yet, just work on understanding the concepts and getting your diagramming. You want to successfully work through each example game, or if you don't get it right, then go back in and figure out why.