I've started to test JUCO's out with my D2 Stanislaus team. I had been using a roster configuration of 4, 3, 3, 2 (FB/FCP). It worked really well, but it does tend to be weaker when you get to the 5 upperclassmen year. So I started to toy with getting one 2-year JUCO every other year when I have 4 openings, in order to boost that weaker year.
So my roster configuration goes like this now...
YEAR 1 - 4 seniors, 3 juniors, 3 soph, 2 fresh
YEAR 2 - 3 seniors, 4 juniors, 2 soph, 3 fresh
YEAR 3 - 4 seniors, 2 juniors, 3 soph, 3 fresh
YEAR 4 - 2 seniors, 4 juniors, 3 soph, 3 fresh
Note that I'm only testing this out with FB/FCP because of the need to go 12 deep, and I feel that by having one roster spot always designated as an upperclassman it will help to elevate those "down" years.