We have a 2022 295MK and often open the slides while still hooked up to the truck.
However, our trailer sits pretty level when on the truck, so it may not be the same as yours and I often lower the front landing gear to stabilize the trailer a bit.
Having said that, we have opened the slides when the trailer is not exactly level (sometimes quite a bit off level). I try to avoid that, but so far we have not had a problem. I'm sure it's best to have the trailer level, but I'm not sure what 'close enough' would be, and I'm not sure what kind of problems you might have if you were off level more than this magic number. I imagine if you opened the slides several times when you were really off level, you could cause the slide to become misaligned or even jam (especially the bedroom slide), but that's just a guess on my part.
Sorry I can't be more clear, just letting you know what works for us.
Well, I have to go now and realign the slides on our trailer (just kidding) LOL
Regards,
Gord