Yes, if your not entering a specific STAR for arrival the FMC will use DEFAULT settings. 
You control the altitude in the FMC. Whatever altitude you enter is what the aircraft will fly.
With that being said,  if you are flying and are going to fly at 8000 ft for example, you need to lower the altitude selector to an altitude lower than the altitude required.

What I do......   If I am at cruise altitude and I come up on T/D, I have to manually change the altitude selector to an altitude that is way lower than the next decent altitude.  If I don't, then the aircraft will not descend.  The same is true if you are required to fly at 8000 ft.  If the altitude selector is set higher than the 8000 then it doesn't matter what the FMC altitude says.