A long time ago, before iOS's 'multitasking', games would often save the entire game state on a suspend event, so that when the app is reopened you can restore the player to the exact position, be it mid-level, deep in the front end menus, or whatever.

These days, for most titles, it's not worth the effort. Just save any user progress as soon as it occurs, if the user suspends your app and tries to return to it, usually the OS will restore your app to the foreground as if nothing has happened. In the event that the OS decided to kill your app to reclaim the RAM, or the user manually killed your app, it doesn't matter much because you will have saved the progress fairly recently.

The only thing I would normally bother doing is to pause the game. One exception is that if your game is pushing the device's memory usage to the limit, then you should make an effort to free a decent chunk of memory on suspend (textures are quite easy to remove/reload), otherwise, the OS is very likely to close your app (particularly bad if your app has links to safari or the app store or something, the user follows the link from within your game, returns to the game only to find it has crashed!).

Here's the relevant doc from Apple: https://developer.apple.com/library/ios/documentation/iphone/conceptual/iphoneosprogrammingguide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html