Stick with the manual. Mine ran no different when changing original plugs at 150ish k miles but they were rather cooked and probably were more stress on the coils than necessary. Better to not wait until it runs bad!
I just did a set on my wifes 2006 Equinox on Sunday. Original iridium's with 140,000 kms. One of the front 3 plugs split in the middle where they weld them together during manufacturing due to extreme rust. I had to get an easy out to remove the threaded portion. This a common thing with 5/8 hex spark plugs. You won't see me ever leave one in again for that amount of time....iridium or not. I ended up putting in a compatible set of NGK platinum's due to availability of the original part number. Runs fine so far.
The 5.3 L engines generate quite a lot of heat. Personally, I would not leave them too long because they can be quite hard to change if left too long. When I replaced mine, the boots were basically welded onto the head of the plugs, I had to destroy the boots in order to get them off. Fortunately I did not break any off during the change, but they were not easy to get out either.
Be warned, if you do this yourself the access to some of the plugs is quite difficult (5.3L). There are some posts on here that give you some tips on how to get to them (particularly on the passenger side).