Our company has several gas trucks and over a dozen diesel engine trucks, from the standard pickup trucks to the big reefer and tanker trucks.
Plus, we have 15 Honda 6500 model generators.
After wildland fire season is over each year (normally by end of October) the trucks and generators sit all winter long until the following June 1st.
We have never put any type of fuel treatment in the trucks or generators.
We have never had any problems with them starting and running.
So I know from personal experience that neither gas nor diesel will be harmed if left sitting for 6 to 8 months..
However I have never kept any type fuel for more than a couple of years, so I can't say how it will fare for that long of a time.
So if it makes you feel good to spend money for those fuel additives, then .. please feel free to do so...
as for me, I spend my money on more important things like, ice cream, movies, fuzzy dice for the windshield....
this is of course, just my opinion as always,