If you have a choice, why not just use it till you see a good reason to change.
my iphone 3GS is running 3.1.3.
I know there are OS 4 out there. Do I always need to update the iphone to latest OS? what will be the benefit of doing so? I didn't update because I don't think 3OS support the multitask function and the new OS 4 will slow my phone down..