The big problems we see with batteries being connected and disconnected is usually when the screen is being unplugged or reconnected. Particularly with older LED based screens, it was not unusual to blow a backlight fuse or circuit. Modern OLED screens like in the iPhone X don't have a backlight so that particular issue is no longer present, but generally you will always want to have the battery unplugged when working on the phone.
But to address your particular concern, no there's actually no problem with the phone being powered up when it's open. I frequently test a replacement screen with the phone open prior to buttoning everything back up, so as long as you weren't disconnecting electronic components while the battery was still connected you should be fine.