I think I mention 5'9 before because that is my height
But truly, it's hard to quantify a cutoff height for when the risk outweighs the benefit. Everyone's circumstances is unique. How much money do you have? What is your current height? How old are you and what's your health (the older you are the more risk you have)? How much do you value being inches taller? Moreover, where do you live?
First of all, average height varies between geography. People living in Asia requires less height to be average than European.
Second is income. You can drastically reduce the risk of complication by going with the top doctor and using the best medical facility that money can buy. For someone with lots of money, LL may be like a hip luxury like buying a personal tanning bed. But for those on low income, LL with a decent doctor could be out of reach and they may have to resort to incompetent doctors, thereby increasing the risk for complication.
Thirdly, how much do you value do you value being taller? I read some comments and discover that there are some members (like Ajax) who would rather die to be 6'3, then to be 6'0. And obviously there are some short people out there who don't care about height. So for Ajax, the risks and complication would be worthwhile (based on his comment) compare to someone who is less sensitive about height.
However, we live in society that is govern by societal norm. Where I live, it is not common for men to undergo cosmetic surgery to enhance their appearance. The cutoff for cosmetic surgery and for therapeutic purposes is the point where the person is doing it to face less discrimination in life which is required (to reach closer to average) and where the person is doing it to enhance his appearance (vanity, domination, etc.) which is unnecessary for "normal" functioning. You can decide for yourself where this point is.
I know that in Korea/Japan, it's not unusual for men to put on makeup and to do cosmetic surgery. But where I live, you risk being branded as sissy if you get caught doing cosmetic surgery. I live in a very conservative part of the neighbour if saying this makes you feel better. However, some people don't care being called sissy or they can move to San Fransisco where being called sissy is hip.