If I have Visual Studio 2008 and 2010 I have
If you open a VS 2008 solution or project in VS 2010 it will convert to VS 2010 and you won't be able to open it in VS 2008.
If that's not a problem, you don't need to save VS 2008 unless you use an addon or whatever another third party application with VS 2008 that is incompatible with VS 2010.
Edit:
Look at Ben Voigt's answer for information on which version supports different platforms.
a source to share
You will need to install VS2008 if you target Windows CE (via Compact Framework, native smart device projects, etc.).
http://msdn.microsoft.com/en-us/library/sa69he4t.aspx
Also, if you want to write native applications that run on Windows versions prior to XP SP3 and Server 2003 SP2, this is no longer possible with VC ++ 2010. The same applies to managed code written for .NET Framework 4.0, but you can still use Framework 3.5 with VS2010 for managed code projects only.
And there is no Intellisense code for C ++ / CLI (MS promises to fix this in the future, be it a service pack or next version, which I can't tell).
a source to share