I recently moved into an apartment where space is at a premium.
Notwithstanding the limited space available, the cistern is hidden behind a wall but is accessible via an access panel built into the wall of the adjoining room.
Visually, the bathroom looks neater, and the access panel isn't that unsightly (you could always cover it with a picture, etc).
Bottom line though is that you need to allow for some sort of access. Some sort of maintenance (even to replace the ball-cock) is likely to be required at some stage.