What I like to do with Cherbourg is to have the units in the area defend off the invasion for as long as possible and even pull back the units that i have left into the city. For reinforcments I dont bother sending up much because of the air cover and also teh long treck that the units would have to go. On avarage I might send up 3-4 tanks if im doing well on the other fronts and a handfull of Inf. But when things get sketchy down south I cut off all the units that would be headed that way. As said before its like a distraction for the allies to help burn time, and maby even weaken the allies.
B