You won’t leak anything – at least in the current implementation.
Calling Dispose won’t clean up the memory used by MemoryStream any faster. It will stop your stream from being viable for Read/Write calls after the call, which may or may not be useful to you.
If you’re absolutely sure that you never want to move from a MemoryStream to another kind of stream, it’s not going to do you any harm to not call Dispose. However, it’s generally good practice partly because if you ever do change to use a different Stream, you don’t want to get bitten by a hard-to-find bug because you chose the easy way out early on. (On the other hand, there’s the YAGNI argument…)
The other reason to do it anyway is that a new implementation may introduce resources which would be freed on Dispose.