...by turning the crown backwards, you are just letting one of the winding gears slip over a clutch. There is probably some very minor wear associated with this, but in a regularly-maintained watch, it is very unlikely to be a point of failure. I wouldn't overdo it (i.e., spin the crown while I'm in a boring meeting!), but this is not a common point of premature wear.
After winding for the day, detail-oriented people (like myself?) can also use this backspin as an opportunity to line up the logo on the crown....
Tom