I could never recommend allowing a water heater to drop below the 120 deg.F. mark for any period of time. If you are gone for just a few days, what is the harm in leaving the tank stay as it is. It is unknown at any time how much legionella bacteria may exist in your public water system at any time. Typically there are low amounts of microrganisms depending on what type of chemicals are being used for water treatment and they have no affect on us because of the low concentration. But don't try to sell yourself into an idea that promotes a lack of safety, err on the side of caution.
Some may say that they have never gotten sick from their hot water, but may have and not realized it. The symptoms are the same as the flu and may take just as long after exposure to show the symptoms. Children and the elderly are especially susceptible to the health risks when exposed. Rarely is ingestion of contaminated water a cause, unless aspirated. It is when taking a shower that it is inhaled with the vapors and settles in the lungs.
The ideas that water sits in the pipes anyways and that temperatures drop in the tank when cold water is used are geared towards ignoring the core issue of safety with your water system.
Just flush the pipes when you walk in the door and keep your tank at the normal operating temperature and you won't have to worry about Pasteurizing your tank or chlorinating the piping.
So, you must keep heaters on the distribution plumbing to make sure the gallons of water in the hot water distribution never drops below 120F then?
When turning off the hot water heater the time window of growth risk is pretty fleeting- once the temp of the tank drops to 85F the legionalla growth rates are about the same as at 120F. Water stored at room temp does not promote the growth of legionella. Do you purge the cold water distribution plumbing too? How about the 70F water coming in from the street all summer in warmer climates?
Even at the prime growth temps it takes weeks or months, not hours or days for a colony to become established. Keeping long term storage temp at 110F-115F to save energy carries some risk. Letting the tank temp fall through those temps down to sub-85F over the course of a day or three does not. If you normally maintain the storage temp at 140F there are no live legionella bacteria in the tank to even START a legionella colony. There can be some live 'uns in the tank if you only keep it at 120F, but nothing is going to suddenly take off in the few dozen hours it takes to drop from 120F to under 85F. The highest growth rate zone is between 90F-105F- above 110F or below 85F it can grow, but it's pretty slow. At 75F or 120F it won't die, but it can't reproduce at more than self-replacement rates. And at 135F-140F colonies are guaranteed to die off-eventually. (It takes higher temps to kill it instantly.)
The original post asked if there was any risks to just turning off a 140F tank whenever the place was going to be unoccupied for more than a couple of days, and the answer is there is none- it starts out sterile, and even if you drew in some live legionella just before turning it off, it doesn't stagnate in the high-growth temperature zone long enough to matter.