I also do them only at tax time. I only want to write them once, and I've had people lose them. I've also had people who when mailed to never get their reciepts (or so they say) so what I now tell everyone, is that since I didn't move that if you need to have your reciept that they can come and get it. I do everyones reciept, put it in the envelope and all I have to do is hand it to them.
I have one family who's reciept is still sitting here, they pass my house every day, but 2 years ago she wanted the reciepts from the year before, so I had to dig thru all my stuff and find it (oh she asked in may so it was way after tax season) so I also have a new rule in my contract that if you lose your tax reciept or ask for the years before then I will now charge you 5 dollars, just like my tax guy does.