Depends on the jelly belly ones in question
The neutradol "ball" ones (white balls) aren't best left under the seat unless you like blue liquid gel all over your floor
The Satchels however are just fine but do note that the air freshener will be used up much quicker if left under the seat because of the floor vents drying it up much quicker.
I don't like those "bug" 3d style ones, the ones with sticky backsides, they seem to leave a residue when removed, maybe they're suited to non matt trim panels or something.
I've switched to using magic tree New Car Smell because they're 99p each and some of my passengers were commenting on my car smelling like a doctor's surgery one too many times with the Neutradol ones!!
I've found an awesome place to hang them out of view too, the knee support lever handle thing, just ha them there and it's perfect
Edit*
For the people saying why people's cars smell so bad to need air fresheners, that's not the reason at all - a pleasant fragrance has been proven to release good moods and is calming. No fragrance is just neutral and a bit boring really.