Well I am the person having the point of view that:
If you crossflash to another fw with the same specifications and do not damage the writer by 1) the flashing and 2) the use of new writing strategies not suited for your original hardware, for me it is not really lying if your writer has a defect lets say with the laser or the spindle motor and you flash back and turn it it.
Some people here hold the opinion that whenever you choose to do a crossflash, your warranty is voided. I think this is true if e.g. there is a dvd writer like yours but with a stronger laser which can also burn vinyl singles, and you flash to that firmware and burn some singles and the laser breaks - you used the device out of it specs.
But example, Philips decides not to update firmware anymore and you go to BenQ firmware (when the hw is exactly identical, of course) and your writer goes defective and you flash back and turn it in - then IMHO you have the right to do that and in many other peoples opinion you have not.
Mainly I would say, whenever you have to use third party software for flashing, it will potentially void your warranty.
I emailed BenQ and asked them about that (example going G to B fw on DW1620 oem). They not even bothered to answer that part of my question! And regarding the question what going from DW1620 G7M9 firmware (latest on EU Benq site) to G7P9 or G7T9 (latest on global site) would mean for warranty, they did the same: Just dont answer.
My official answer is: Only update to the latest firmware on the localized website of your writers producer. That is the safe way.
My inofficial answer: As they are sometimes fsckin lazy or not caring to put up more recent firmware, do whatever you want but stay in the specs. If there is a problem and you CAN cross-flash back AND you are sure it was not the new firmware (like 4x DL writing instead of 2.4x - that already MAY be out of specs!), RMA - and good luck !!! This is possibly illegal and may even get you into court.