Like with so many other questions, to me the answer is "it depends"!
Back when I was getting started with Photoshop, the "accepted wisdom" was: save you files as 16-bit tiffs/psds with all your layers intact. I accepted that advice without much question. Of course, at the time we were in the early days of digital imaging, so our photos were either scanned photos or early digicams that produced low-MP images and so a tiff/psd wouldn't be as overwhelmingly large as they have become.
But, I also have seen things come up over the years that have challenged that conventional wisdom. Especially with the advances in Raw processing, the "16 bit rule" has been getting a new look, and in fact a lot of us have "adjusted" our thinking there. Also, a lot of us have adopted Raw processing as our "main" editing process, so we don't often resort to Photoshop, layers and such.
But, as has been mentioned, there are times when you want/need the PS tools, there are times when 16 bit files in PS can be good, and certainly there are jobs done with layers that you invest a lot of work in and that you don't want to discard.
I did watch an interesting little video by a co-worker of Scott Kelby once. He was buzzing through processing shots in Lightroom, then opened them in PS, tossed up some layers, and quickly finished the job! Funny, at one point he flattened most of his layers, and I was like "what the heck?"...but as I thought about it I realized that the work he had done with those layers was stuff that was very familiar to him, no-brainer type of quick stuff, and so to him there was just no interest or need to revisit them.
Now of course he was an experienced "expert" and could get away with that. Perhaps in the learning/beginning stages I would, well, give some thought to my actions!