It probably would be better to make the objects global and use (boolean) instance variables in that case, for global variables would clutter things up..
It all depends on how your game is set up..
Using an (global) array to hold all the (visibility) values would be another option..
Also if you wanted to create a level-unlock-thingie, there are multiple examples in the forum and one or two tutorials in the tutorial section..
It would involve adding to the global variable and checking if level number is less than or equal to that variable, I guess..