i'm on the verge of getting a new 27" imac. was thinking of getting one with only 8GB of ram so i can update myself for less than what apple charges. i have a feeling this would void the warranty. hopefully i'm wrong.
Also if upgrading the RAM yourself is important to you for $ reasons (like me) and you were thinking of waiting until an iMac 2018 is announced (maybe, see other threads), then note there is _speculation_ that a new iMac may remove the “door” for users to upgrade the RAM themselves on the back of the 27 inch machine. Don’t think anyone’s seen any evidence of it either way but with Apple’s other models this has been the trend.
Likely would not void the warranty, but if you start having RAM type issues with third-party RAM installed Apple will have you remove the RAM and see if it can be replicated without the third-party RAM. This is their step to identify if it is the RAM or something more.