Hard to say really. From what I've learned so far I do this (cold pressures):
The faster the track is the higher pressures I use. For example, at fast tracks with minor elevation like Spa, Okoyama, Motegi or even Watkins Glen, you can use even between 180-200 kPa. You'll feel the curbs more and the car doesn't respond so well, but there's is not so much tire wear I'd say. I can't really explain it properly since I don't fully understand it, but somehow higher pressures seem right at these tracks.
On the other hand technical and elevated tracks like Mid-Ohio are better with lower pressures since you need loads of grip. Lower pressures provide larger contact patch (notice however that the curve is not linear, the less pressure you have the warmer the tire gets -> the pressure rises). I used around 140 or 150 kPa this week, I am not sure right now.
I noticed an interesting trend. When I use lower tire pressures, sliding seems a bit more "right". For instance, during my race at Mid-Ohio, I experienced an amazing slide at T8 and T9. I didn't manage to save it, but anyway, it felt like a proper slide on a real road. On the other hand, hwen I use higher pressures, the car seems to be more prone to spinning and I can't save slides so often. But as I said before, I would worry about it too much. I often use baseline pressures since they usually fit the track and I am no alien so I can't really feel small changes. What I noticed is that I usually make the pressures lower, not higher.
I am waiting for the next update - I hope they'll intoduce flat spotting and correct the tire grip curve, it might solve a couple of things (in my head at least).
If you wish, I can post a video of this interesting slide (well, not really interesting, but it's interesting in terms of iRacing's physics). I can also provide Wtkins Glen and Mid-Ohio setups which worked well for me. At the moment, drivers are confused a bit and I don't think anyone knows exactly what's going on with our tires.