It is important to do those PT under timed conditions. However, what you can do is find the sections where you take less than the alloted time. Use those extra minutes and allot them to another section where you know you are weaker. For instance, PS usually takes less time than DS so you can allocate the time as I said. Otherwise, PT, I would say, is fairly representative of your potential score if not even a bit higher due to repeated OG questions contamination. In any case, the most important is that you use those as practice and do not get too deterred by your score because even though you get 750+ in those tests, you could still end up with only 650+ on the actual test. Same thing is true for getting higher actual GMAT scores than paper test scores.