I'll just add my 2 cents, since I've been using funtoo for some years now, and gentoo for some more.
After the main installation, I install all applications which I can think of at the moment and add more as I use the system. During this "phase", I have to deal with USE flags a lot more that I like.
On my desktop system, which is my main machine for experimenting/trying out stuff I reinstall every few months, since I a) like to test out other linux distributions to see what they offer and b) to match the current installations process with the installation guide in the wiki (and to update the guide, if needed). On my laptop, I need a stable system for working - like drobbins - so I have only installed funtoo there once.
The debian-sources kernel works fine for the most part, however it doesn't support the keyboard of my Laptop (a Surface Pro 4), which is why I'm using the gentoo-sources kernel there. Also, on my desktop I sometimes experiment with kernels like pf-sources to see if I can get better performance in games, but that never happens ;) kernel optimization is a really time-consuming process.