That's a hard one to answer. I've long been a fan of just maxing out whatever you have, but 64GB is pretty excessive for normal humans... that said it's getting older and probably harder to get so grabbing it while you can is probably a good idea... but even so 64GB is still probably not worth the investment unless you really do things that would use it.
From memory my old 2009 17" MBP was upgraded to 8GB RAM and I was concerned about dropping down to 4GB when I upgraded that to a 2011 MBA. Turns out it worked just fine, even for the VM's I was running, although that was probably down to the SSD more than the RAM. These days I'm running a 2013 MBP with 16GB RAM and it's more than enough for me and I can't see that changing anytime soon.
In your case, there are a couple of factors. How long will you keep the machine? Will it keep running the latest version of OSX? Do you do anything that needs more RAM?
What have you actually got in the slots at the moment?
2 x 1GB + 4 x 2GB = 10GB
2 x 1GB + 2 x 4GB = 10GB
6 x 1GB + 2 x 2GB = 10GB
I think that just grabbing two new 8GB modules (16GB) and adding that into your current mix will give you more than enough ending up with 24/26GB of RAM depending on your base config.
2 x 1GB + 4 x 2GB = 10GB => 2 x 1GB + 4 x 2GB + 2 x 8GB = 26GB
2 x 1GB + 2 x 4GB = 10GB => 2 x 1GB + 2 x 4GB + 2 x 8GB = 26GB
6 x 1GB + 2 x 2GB = 10GB => 4 x 1GB + 2 x 2GB+ 2 x 8GB = 24GB
Grabbing 4 x 8GB modules does give a small saving over upgrading later (shipping aside) and would push that up a little more but is going to set you back a bunch more for questionable gains...
If you haven't already, invest in an SSD. SSD's make everything better. Given space isn't an issue a smaller SSD + larger spinning drive for storage is the best bang for buck. 240GB SSD's are dirt cheap these days, even 512GB drives are pretty damn cheap too. I mean you could get a 2TB SSD, but I think that's only worth it if you are doing something that really needs that much capacity and speed or if you had a portable that can only accommodate a single drive.
You could go all out with PCIe storage solutions for that little extra bump, but honestly, SATA is probably more than enough for this machine.
It's worth adding up the cost of all the upgrades vs the change over cost to upgrading to a 2012 Unit (being the last model refresh before the trash can) remembering that you will get cash back for your current unit. Chances are you can find a 2012 machine with all of the upgrades already installed into a faster machine that is natively supported by High Sierra.