I don't warm-up read, but I definitely choose when to read what.
I read the slop when I'm bored and want some quick fun; the serious stuff, I read it when I have some quiet time to concentrate fully.
Though I tend to first start with the slop to get in the mood, then stop because it's boring (who would have thought?), then I try the serious stuff after a break, so I might actually "warm-up read," just with a break in between...
When I'm reading seriously, though, I don't warm up the next day; I just continue the story where I left it.