I add fuel stabilizer then top off the tanks preventing condensation from forming. Adding the stabilizer before fueling then a run over to the marina helps make sure it is well mixed in and in my opinion gets it through the entire fuel system.
You can go both ways but make sure to tape or seal fills and vents because E-10 will attract moisture from the air. I emptied mine put stabilizer in and taped fills and vents. Did I post recently how much I hate E-10, 15, 30!!!!!
We are fortunate to operate in an area that has plenty of no-ethanol fuel sources which allows us to NEVER load any ethanol enhanced gas in our tanks. Also adding Marine Sta-Bil 360 each fill-up per directions has yielded great results. I keep my tanks topped off year round.
I read somewhere, possibly Consumer Reports, that almost all fuel additives/treatments are nothing but a scam. Think about it, how can a pint of the scam remove a quart of water from gas. I know the labels talk about breaking down the water so it can get through the fuel system; I am chemistry dumb, but how can water be "broken down" so it will go through a fuel system? Snake oil for sale. Don't use E-10 gas, ever.