Should be some feet away as the leaves are quite large, the clumps - while dense - still do creep sideways. And bold looking plants do not work well as background features.

this is how i look at potential of a plant.. glad you provided the latin name ...

https://duckduckgo.com/?q=Musa+basjoo&t=ffcm&iax=images&ia=images

sure doesnt look like a couple inches from the fence is enough ... ken

I would plant them 4 feet from the fence. Still the leaves will eventually hang over as the leaves get up to 7 feet long.