I know that Mrs Flynn has already answered your question, pheonixc, but here's my experience with the Expansion and Stuff Pack Manager.
I have all expansion packs except for seasons, pets and island paradise installed. The only stuff pack I have installed is High End Loft Stuff installed. When I go to play, I generally tick all of the boxes in the expansion and stuff pack manager except for into the future. i have access to all of the content from the installed and selected stuff/expansion packs in my game.
On the odd occasion when the selector decides to deselect my expansion and stuff packs, worlds related to the expansion packs (eg Twinbrooks) aren't available. And if I have a saved game set in one of those worlds, I'm unable to open those games. (This is how I know when the game selector has deselected the expansion packs I have selected).
The expansion/stuff pack selector has no impact on downloads from the Sims 3 store. So, regardless of what expansion/stuff packs are installed and selected, I always have access to my Dragon Valley world.
I haven't run into the issue oi sims being sent to a holiday world/uni/into the future in a saved game, then to have the expansion/stuff pack manager decide to deselect the related expansion pack, but I'd imagine it would probably grey out the saved game image and not allow you to play it.
As I said, that is my experience with the selector, hope something in there is helpful to you.