Memory for new iMac 32gb or 64gb ... Thoughts?

I agree, most experienced users on these forums only use Crucial.com or www.macsales.com RAM. Both are dramatically less money than buying direct from Apple and installing RAM is a 5 minute process that requires zero tools. The reason for recommending these 2 vendors is iMacs are sensitive to using quality RAM and both vendors have long histories of supplying quality products, excellent service and fair prices. Apple price is $1400USD for 64GB, www.macsales.com is $549.88, the difference is dramatic and funds extras that will be needed such as Time Machine Backup EHDs (again use MacSales.com) to get the Mercury Elite Pro Series. Do not purchase cheap Seagate or WD enclosures.

To answer your questions, most users are fine with significantly less than 32GB, how much you should get it really up to you. As you are not sure, get as much as the budget allows.

Lastly, OWC had great install videos - assuming this is the model you are going to buy:

https://eshop.macsales.com/installvideos/imac-27-inch-5k-late-2015-memory/

I'd also think that 32 GB would be plenty; if you're buying it with the standard 8 GB, you could actually get the 32 GB kit (2 x 16 GB) which would get you a total of 40 GB. That kit is 279.88.

If it were me, I would order it with 8GB and upgrade to 32 or 64 GB. Third-party RAM will save quite a bit.

I second rkaufmann87's recommendation regarding purchasing the memory modules from MacSales.com or Crucial.com. It will cost about ½ of what Apple will charge. Being a 27" model you're ordering you can do it yourself.

For example from Crucial you can get two 8 GB modules for $131 for a total of 24GB. The 16 GB option thru Apple is $200. Or two 16 GB modules (for a total of 40 GB) for $258 vs $600 from Apple.