There once was a time when agents would freely leave their business cards when showing property and many still do.
There have been many changes in the code of ethics for real estate agents and many activities that were once seemed commonplace are now in breach of the rules.
Leaving your business card, unless requested in the showing instructions, could be construed as soliciting the seller's business, which is a clear ethics violation. Your business cards aren't doing you any good if you hang on to them but stop leaving them for individuals who are already under contract and represented by another agent. Save your cards for someone who could use your assistance rather than someone that could land you in trouble with your real estate commission.