It's not just the slope, it's how fast you are going and and what you are doing. Just after I got my CUT I almost rolled it on a 10 degree slope by being a hurry and turning too fast. Those uphill wheels came off the ground and scared the hell out of me. Just after that I got my tilt meter and I was surpried how closely my butt and the meter were calibrated. At 15 degrees I was getting uneasy and at 20 I was looking for a way out. Having said that, I occasionally get a guy to cut my pasture and he runs across slopes that would terrify me.
Having looked at the photos and I know they can be deceptive, I think I would be comfortable mowing them straight up and down but you'll get a better idea when the tilt meter arrives. Just remember, if you feel uneasy, don't do it. There are a couple of small places on my yard that I still "trim" with a walk-behind.