The higher the framerate, the lower the latency. iRacing's unforgiving physics need as much responsiveness as possible, so if you are comfortable with it being uncapped then there's no reason to play it any other way (unless it's really jumping around a lot).
For me however, any framerate that doesn't multiply/divide nicely into 60Hz looks really odd on my screen. So only 30fps, 60fps and 120fps look smooth. Somewhere halfway between 60 and 120 looks no smoother than 30fps in my eyes, even though it's clearly more responsive, so I stick with 60. If I could maintain 120 all the time I'd do that, but I can't, so it has to be 60 for me.
I used to play everything with v-sync enabled for this reason, because it would force a 60fps cap. I'm so sensitive to this that I thought disabling v-sync actually reduced performance, because the higher framerates looked less smooth. It was quite a revelation when I realised this wasn't the case, and I was actually getting better fps without it, and I just needed to cap games manually to 60fps to give me the smoothness I enjoyed with v-sync while massively reducing lag.