In theory...ALL readings COULD be skewed by an uneven ISS.....
The most sensitive would likely be the rain gauge......the tipping bucket really needs to be level to "tip"
when it measures each .01" of rain....
How high are you/have you mounted the ISS??
Mine's at 15' AGL....to level it, I took a small mirror and mounted it (temporarily...) to a snow rake, hen raised it up so I could see the bubble level and apply the needed tension to 3 nylon guy lines just under the ISS mount.
The key is being able to see the bubble level; if you install past ladder height, it's either guesswork....or improvise.....!
Which is why Davis recommends around 10 feet, start going over and your rain and temps, dew, point etc will be out, but wind would be more accurate. also for access and getting this level 10 feet is ideal.
I have mine at 10 feet, it's a good compromise, also don't need guide wires, and access is easy and see the bubble level.
The pole needs to be solidly in place as flex will give false rain readings, hence I have only gone 10 feet. Compared to other weather stations where I am, the wind direction and speed seem ok.