As far as I know, Apple does not "assemble" RAM, but simply buys it from manufacturers like Samsung or Crucial and generally charges several times the going market price. For example 16GB of RAM (8x2GB sticks) costs $3500 if purchased from Apple, the same 16GB from Other World Computing (http://eshop.macsales.com/shop/memory/Mac-Pro-Memory) costs $850.
One difference between RAM purchased from Apple and that purchased from OWC is the warranty:
Apple RAM has the same warranty as the computer - 1 year or 2 additional years if you also purchase Apple Care.
RAM from OWC has a lifetime advance replacement warranty - - which means that if you have a bad stick, you notify OWC and they immediately send you a new stick and do not charge you unless you fail to return the defective RAM within a specified time period.
I have purchased RAM over many years from OWC and have NEVER encountered a defective chip.
It would seem that Apple spreads negative propoganda about RAM from 3rd party vendors simply to justify its outrageously exorbitant pricing.
Save the RAM that comes with your computer. If, at some point, your computer needs to be repaired by Apple, remove the 3rd party RAM and reinstall the RAM that came with the computer. If you don't do, this Apple will claim that, whatever the problem was, the 3rd party RAM was, at least partially, to blame. Thus, if the power supply should fail or the liquid cooling system on a G5 PowerMac develop a leak, or a hard drive die of head crash, Apple will blame 3rd party RAM even if, according to the known laws of physics, defective RAM could have no possible connection to the problem. Again, this appears to be a a way of attempting to make consumers fearful enough to pay Apple's exorbitant prices.
Similarly, Apple does not manufacture hard drives, it buys them from companies like Seagate, and charges almost twice the going market price. For example, each additional 500 GB drive (manufacturer unsepecified) costs $250 when purchased from Apple while a Seagate 7200.11 500GB drive costs $135.00 from OWC and a Western Digital 500GB RE drive costs $115 from OWC.