Best way to add "Check Date" to a contribution?
I've always just added it as a custom field.  While I agree that it'd be preferable to not have it appear for non-check payments, I've never been asked to implement that.  That could be accomplished with one line of jQuery though, which you could drop in an extension.  If you don't have experience building extensions, you can also use an .extra.tpl file, which is less work.