I have maybe 3 cannon fodder and 5 valuable soldiers in an average team of 8 in total, but it depends on the mission also. Some missions are without any civilians, then I may have only 2 or even a single cannon fodder. Other missions are pretty hasty, such as subway or city high rise, then I have maybe 5 cannon fodder and only 3 valuable.
The goal of cannon fodder is to minimize the losses of valuable soldiers. I lose maybe only 3 or 5 of them in a whole ingame year with maybe 100 missions. It is probably different on lower difficulties, but on very hard in 2.6 a single attack does absolutely sufficient damage to kill anything, no matter how experienced or armored the soldier is. The only important point is whether the shot(s) hit, and I don't know whether they will, so I cannot risk putting someone valuable onto that position. It's better a cannon fodder soldier dies with -1000 hitpoints than a valuable soldier dies with -100 hitpoints then, so to say. This absorbs quite a lot of the alien TUs, as they love to use expensive shot modes (full auto) albeit snap shot would suffice. And I do not need to spend TUs to heal the bleeding, if a (valuable) soldiers survives.
The good thing is that not wearing armor grants you the great TUs bonus, and even cannon fodder can easily kill a powerful enemy when on close range. It's always difficult to decide whether to give the kill to a unit it's wasted on, but the cannon fodder may well be dead the next turn and that enemy is still not even wounded.
While cannon fodder is not very valuable from a campaign point of view, it is from the point of view of the current battle, because once all cannon fodder is dead, I can no longer take risks; such missions are awkward then. So I put even cannon fodder only into danger if there's a chance the soldier survives, or if there's something to gain, such as pulling shots away from civilians or luring aliens out of cover.