Apple reps, and I have had this experience even in an Apple retail store, do not tell the customer they cannot do something the company literature, pointed out by babowa, says they can do. The caveate is that if you break it, Apple voids your warranty. But it is very hard to damage a Mac just by putting in ram if the ram has been properly manufactured.
Macs are extremely sensitive to the timing characteristics of the ram installed. Ram may have the same "specs" as Apple says, but those are only the highest level specs, such as the DDR class, clock speed, etc. The less familiar characteristics such as latency (timing between receipt/refresh for data) are not the same for all ram of a high-level class. PC ram may state the same high level spec but be way off on things like latency, and then not work in a Mac. It is not the Mac's fault, it is a matter of ram that does not satisfy the Mac's critical needs.
A number of years ago Newegg was a great Mac third-party supplier, I used their products in several Macs. But lately there have been repeated complaints in these Communities from users about ram purchased from Newegg.
The one company there are rarely complaints about is OWC, as stated repeatedly in this thread. OWC offers mutltiple ram choices for most Macs, including the same brands as used as OEM supplied ram. If you are concerned, talk with the OWC help desk, explain the situation, understand their return policy, and order the Apple OEM ram for your iMac. It is more expensive than the other OWC offerings, but still much below the Apple price for the same ram.
