It is fairly important to have a site management game plan. Letting Civil 3D put everything on the default site will get you.
I was adding a proposed county forcemain to my design so I can get it on the cross sections. I created a Feature line, and let it assign this to the default site named, ‘Site’. I just needed some quick feedback. Once I applied elevations to the Feature line, the EG surface went ape. I reviewed the surface etc. but no joy. Time to think logically.
Sites are made for interaction
In the image above you should see that ‘Site’ is populated with numerous grading Feature lines. By permitting Civil 3D to apply my FM featureline to this default site, it permitted the interaction of the forcemain with the other Feature lines. It was not pretty.
I fixed it quickly by moving the FM to a new site named FM, and the surface cleared up immediately.
It is a solid idea to develop some good Site management habits. Whether Parcels, Feature lines, Alignments, etc., get a game plan for which sites should do what, and what to do when new Objects are created.
It might be a good idea to stop letting Civil 3D apply the default site. Even if the object created is temporary, it may conflict with others that you have forgotten about, and may not be evident for some time, especially if various displays are turned off.