It's been a while since I had it open loop, but I do think I remember it moving around a little bit. That might have also been one of the reasons I started working on the closed loop idle.
How much is yours varying? I think one of the tricky parts with the open loop setup is that it only references coolant temperature to control the IAC. I think MAT has some effect on that as well, though maybe less so if the correction curve is set up well. So your coolant might be at 180 degrees or something in the middle of winter with an IAT of 40 degrees, or the middle of summer with an IAT of 140 degrees. On a smaller scale, it could be like exiting off the highway where you've been running fairly steady state for a while and might have stable IAT values, but as you drive through town in more stop and go conditions, that IAT rises.
I was really struggling with my idle after I installed my A/C because I was getting a lot of surging. I finally got a dashboard set up so that I could watch the IAC position while things were running and it helped me better understand what was going on. Long story short, my target IAC position and my starting position were so far off that the PID control was having a hard time getting things settled. I probably could have kept playing with the numbers to get it there eventually, but I found it easier to just fix my starting position to be closer to where it wanted to settle. I still get a little surging if I turn A/C on real soon after starting the car, but it's typically pretty stable within a few minutes of running.
So speaking of IAC starting position, that might also be what could be causing an issue. I'll probably go over it more when I get to the startup/idle settings tab, but there is a setting for the IAC to "use last position" or "use table". I don't recall if that's only when you choose closed loop or not though. Regardless, I'd suggest making a gauge/readout that displays the IAC position and checking it as you drive around and notice your idle changing. If the IAC position is the same all the time but you're getting rpm changes, I'd wonder if maybe it's a throttle linkage issue like the blades aren't closing all the way? If the IAC position is moving around, then it's more just a case of figuring out what setting is causing that and seeing if you can update and tune it.