Is it advisable to upgrade my 3gs iphone to the 4.02 firmware?

I'd say no. Just in case you want to sell your iphone in the future. Someone may want to jailbreak it to use it on T-mobile for instance. 4.01 is great and can be jailbroken but 4.02 can't as of yet. If you really don't care about jailbreaking or selling your phone to someone who wants to jailbreak, 4.02 is virtually identical to 4.01 with the exception of the pdf flaw has been patched (which blocks jailbreaking)