Some people have had issues with the first versions of the 27" 5K displayed iMac's running with more than 32 GB of RAM. So if you have one the first versions I would recommend you stick with 32 GB even if someone tells you it can be boosted to 64 GB.
Heavy RAM use can be due to what you are doing or it can be caused by a misbehaving app or two. So I wouldn't jump on putting more RAM into a system until you've verified you really need it.
How about running Activity Monitor app which is in your app utility folder as well as look at a good memory monitoring app like: Memory Clean2. Can you give us a clue on what you are doing as we might have some ideas how to be more effective here.
I do recommend having a SSD as the boot drive and making sure it is not over loaded with junk (at least 1/3 free). While this seems to be outside of RAM it really isn't. You see your system leverages the boot drive for virtual RAM and caching/paging within the app. So having fast and available space on the boot drive is important. While some people use a Fusion Drive we've moved to dedicated drives (basically, breaking the fusion drive set to there discreet drives). Size of the boot drive can also matter. Someone working on large images. video editing or music composition needs more space.