One of the official purposes is to test the docking. It seems unlikely that this could be considered complete if the hatch is never opened. And if the hatch is opened, odds are very good the astronauts will enter it at some point in time, if nothing else then for a PR moment.
Astronauts routinely enter Cargo Dragons to stow return experiments and trash. (I do not mean to imply that Demo Mission 1 will return any ISS materials. Have not seen any info on this.) Since this is a (very expensive) test flight, it's virtually inconceivable that a visual inspection and video record wouldn't be made, in case the craft is lost on return.
The reasoning of PearsonArtPhoto is sound. Once the hatch is open, it is no riskier to be on the capsule side of the opening than on the ISS side.
Perhaps not definitive, but I saw this in an NPR News story today:
The three astronauts currently living in the outpost will be able to open the hatch and go inside to load and unload cargo before the capsule returns to Earth and splashes down in the Atlantic Ocean.