I do both, not systematically, but pretty much all the time. Often, when new CAS or BB content is released, there will be something that I really want for a particular build or a particular sim, then I go into that household or lot and add it in. Sometimes I will even do a complete makeover. I may add that I do this with CC too.
I tend to have fads and obsessions, though. For example, one of my longtime obsessions is curtains. I have never ever liked the base game curtains. The simple curtains that came with CL were a great improvement IMHO, so I used them to replace a lot of the BG curtains in my previous builds. Then the Seasons curtains came out and I liked them even more, so they are pretty much all over the place now (along with some very nice CC recolors that I have found).
