I would do it timed, then if you don't get it in the time allocated don't look at the solution just try and work it out and time yourself doing so... when you think you have it see if it matches the solution.
You will get better and you should still get an accurate appraisal of your current rating.
Is it more helpful to do the TT timed or untimed, in your opinion? I'm doing it timed right now, and am pretty much staying around the same old crappy rating. Would it help to do it untimed, or would it just be a waste of time?