Same as Cadillac. I have the blurb in my contract but have never enforced it. I have only had it happen once. I didn't even realize but the Mom called me horrified that her check had bounced! They brought me cash the next day and continued to pay by cheque without any problems.
I think you can use your judgement. It will become obvious if it is more than an honest (embarrassing) mistake. Put the blurb in just in case but if people are repeat offenders you have bigger issues than a NSF fee!!