Define offroad. Rock crawling or any other option that will have you dragging those tires over sharp objects such as rocks, stumps, logs, ledges, sticks, twigs, yadda yadda etc etc and you might want to find a good LT tire. Checked out your local Walmart? Their Super Sport HT tires go pretty cheap.
Anyways, if you're seriously planning on abusing the tires I say save for the LT tires. If you're just going to be bombing down 2 tracks, playing in known safe mud, sand without a lot of jumps and hard landings, etc you should be fine. I had a set of P-rated 235/75R15 muds on my last Tracker and they held up well. I kinda wished I had spent the extra $60 and gotten the LT rated tires but they worked well for me. Will this Tracker's next set of tires be LT rated? You bet. The difference in sidewall stiffness is amazing from experience with other vehicles.