SHSH blobs are vital information unique to that specific device and will be needed in order to restore back to that specific firmware (OS) EVEN after Apple has stopped signing it. which Apple has stopped signing 5.0.1 a long time ago
situations where you might be forced to upgrade could consist of getting a hold of a bad tweak that corrupts your device, or unknown battery drain etc. and sometimes issues not even related to jailbreaking
more than likely you probably will be able to stay "that way" until you get a new phone but nothing is for sure. you're dealing with software which can go south on you at times
one scenario for you could be as such, lets say you do not save your SHSH blobs for 5.0.1 (which you can anytime as long as you are still on it) and then Apple eventually stops signing 5.1.1 (which they will) and for some crazy reason you have to restore your phone. you will be forced to update to the newest OS which more than likely will not be jailbroken at that time. then you will have a nice stock iPhone for who knows how long