With jellybean I have never flashed a single ROM on my nexus 7. Its not even rooted or boot loader unlocked. Stock is almost perfect in terms of speed and UI. No need. Perhaps a bit more customisations but I can do most of that with some useful tools/apps from the play store.
On the galaxy nexus, I did not flash nothing at first but then I just tried a few ROMs just for fun. Most were quite cool. Some nice additions. Blackbean and rasbean jelly had some nice touches. But I had problems with battery life on one and a reboot issue on another. So I been on stock ever since. Unrooted too...just to keep my sky go working sweet!
On my old devices however, such as the galaxy note and Asus transformer, I used to flash every now and then. Not as crazy but perhaps every few weeks. Once I found something I liked I usually stuck with it.
Edit...I mean stock 4.1 on the galaxy nexus that is. Don't even go there with 4.2. Sad story for me.
Sent from my Galaxy Nexus using xda premium