During my preparation for the LSAT I tended to answer all of the questions in the time allotted to me.
But occasionally, especially in the beginning of my prep, I would be left with a question or two unanswered. I always guessed. Why? Because you're allowed to do so on the test - so really, the question is "Why not?"
IMO, it is useful to practice guessing in itself. For example, I would quickly eliminate answers that were completely off and make a best guess between those two or three plausible answer choices. This is not a skill you want to be without when sitting for the real thing.
Also, make a mental note of which answers you guessed (or physical notation, such as an asterisk, etc). Then review the question more carefully during your postmortem.
Bottom line: for a first diagnostic, guess. Chances are, you'll have to guess one or two answers in Sept anyway.