I use a combination of Design Center, the _ImportStylesAndSettings command, and drag/drop from other drawings. What I like about the _ImportStylesAndSettings command is that you can choose which styles get added, removed, updated, etc. Also you can import the command settings with this option.
I actually wrote a short blog about it a few years ago. You can find it here.:
http://crossing-the-line-with-c3d.blogspot.com/2011/12/quick-tip-about-importing-styles-and.html
If you just need to import the object layer settings, here's a blog about how to get this from your template into your current drawing.:
http://crossing-the-line-with-c3d.blogspot.com/2010/08/transfer-object-layer-settings-between.html
Regarding inserting a drawing with Civil 3D objects into a template then exploding: I find that when using this method the Civil 3d objects get renamed such as a surfaced named 56101-EG becomes 56101-EG (1). If you try to rename it the original name you get an error message: "The name you entered (56101-EG) is a duplicate. Please enter a new name."
