If you are using Safari as your web browser, it would be best to have as much RAM as possible until and unless Apple fixes the terrible memory leaks. Safari uses up 10+GB for me, unless I restart it regularly.
Also, the system will use free RAM for filesystem caching, so it is never really "wasted" unless you have a ridiculous amount. 32GB no longer should be considered ridiculous.
All that said, since you already have the 8GB, I would suggest going to 24GB, as you can use the original RAM. If you later feel you need a little more, you can always add another 16GB and remove the factory 8GB.
While I can't talk for other people, for a week I had only 8Gb RAM (while I waited new RAM to arrive). Then I had 24Gb, and in the end, sold back the 8 to have 16Gb.
In all this time I didn't notice any difference. I work mainly in Photoshop and I always have Safari running with a few tabs open, iTunes, Evernote, etc.
Once again - I noticed the exact same performance with 8Gb as I did with more RAM. Also, just out of curiosity, I tested 16Gb RAM to see if it was enough (compared to 24Gb), and I couldn't start using the swap file even with Safari (numerous tabs), Chrome (also), Photoshop with 4 large images, Evernote, iTunes and Sketchbook Pro open at the same time. The only thing that happened is that at the very end of opening all this stuff, memory started getting compressed. The iMac was just as responsive as it was before opening things.
Now, I don't know how many tabs people have open, but I just couldn't get the computer to slow down with 16Gb no matter how much I tried. I also had no issues with 8Gb.
I keep hearing about people having memory issues just while surfing, but I find it hard to believe 8Gb RAM is not enough for that. Let alone 16. 24Gb I don't even want to comment.
Also, as you mentioned, the applications can indeed use free RAM to cache their data so that it doesn't have to be recalculated or generated again, etc. This of course speeds up things, however, the application does need to actually have things to cache. It's not as if the free space gets filled by itself by moving files to some imaginary RAM drive. Even 8Gb should be plenty enough for applications to cache their data while you're working and keep it there long enough to be useful before the memory gets reclaimed by the OS. The good thing is, you don't have to guess - all you need to check is the
memory pressure in the activity monitor. That's why it's there for. As long as it's green, you have no problems. And it was always green for my regular workflow with 8Gb RAM (Photoshop, Safari, Evernote, iTunes) no matter what I did - and, to confirm it, the computer felt snappy and responsive with no slowdowns. It felt
exactly the same with more RAM.
TLDR: I stand by my claim - 8Gb RAM is enough for most tasks, including demanding ones, 16Gb is just breathing room and nice to have, 24Gb is serious overkill for all but the most demanding users dabbling in video and rendering.
Seriously, this debate is an old one. People keep finding reasons to add RAM and then (as a result of what I'd call an IT-placebo effect) they think the computer works better. Of course, there are more than valid reasons when even 32Gb RAM is not enough. But to claim that "pros" require 24, 32 or more is just not true.
Also - I always direct people to this great video by LinusTechTips.
https://www.youtube.com/watch?v=ajyzZ-zaq0o