Should we be concerned when people store assignments on the site?

No.

Update: I suppose I could elaborate, we shouldn't be concerned about assignments being stored on the site because:

• False positives. Who's to say if it's a homework assignment?
• Other's can learn from the code.
• It would be more work for the editors
• If a few people want to use it instead of doing their work, it's their loss, doesn't bother me any.

So, in conclusion, No :).