Yes, definately in lawn mowers. If your weedeater takes mixed fuel (oil and gas) its not a big deal because the ethanol in the gas doesnt draw out the moisture because of the oil present. 2 strokes aren't affected nearly as much as 4 strokes. Also, fuel injected motors don't seem to have near the issues with gumming up from ethanol as do carb'd motors.

To summarize: if it's 4 stroke, or carb'd, you definitely need to add fuel stabalizer. 2 strokes can get away without it, but its still a good idea to use it if you don't use that engine often, particularly if you use gas that has ethanol in it. Stabil is a good one to use.