I think fleece provides very little warmth. Richard said about 1 clo for 100 wt. I don't think heavier fleece provides that much more.
If you're hiking and it's very cold so you need something more than base layer and jacket, then fleece is good. Doesn't provide much warmth, but that's what you want or you'll sweat.
It all goes back to Richard's chart http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/9378/index.html
For moderate pace walking, 1 clo good for 50 F, 2 clo good for 10 F. Base layer, jacket, trapped air layers, boundary layer on the outside is maybe 2 clo. If you add 1 clo, then you can do 40 F colder.
For sleeping, 1 clo adds maybe 8 F, hardly noticeable. If you have fleece anyway, you can wear it while sleeping but it doesn't much matter.
I think sometimes the term clo is used when really iclo is more precisely correct. And, of course, if you're talking about what insulation to put over your entire body they're the same - if you say 1 clo is good for 50 F, you need 1 clo on your head, 1 clo jacket, and 1 clo pants.