Don't be lazy with cleaning up after yourself! Even in C#. Just because it has a garbage collector, doesn't mean you can be that lazy. Always call Dispose() on IDisposables (including MemoryStreams, and ESPECIALLY Bitmaps), and I'm in the habit of setting everything to null when I'm done using it (not sure if it actually matters though; performance-wise). There are still SOME memory leaks in C# to watch out for.
<sarcasm>I set everything to null 5x just to be sure, and terminate all lines with 5 ;'s just in case.</sarcasm>
Disposing is important, yes, but setting stuff to null? Pointless noise. Stop it!