Yes, it is true as AVITWeb says; there is an option in the 'transfer settings' to instruct SS to delete the converted files after transfer.
However, i'ts still always a good practice to take a look in that default folder or follow that file path because, in my experience, the deletion doesn't always happen --- for reasons that remain a mystery to me. {I guess computer programs sometime ignore instructions and do want they want to do just to defy the humans.}
Don't forget about clearing your Recycle Bin either, if you do have to perform a manual deletion. My two cents.
Happy Transferring,
-ja