I do it both ways using 505 spray.
I prefer the fabric, because I have a better sense as to how much I have sprayed and where.
It disappears more on the batting, and so, not as easy knowing where it is.
Plus I feel it stays on top of the fabric, ready to stick, instead of sinking in on the batting.
Of course, some of this is mind over matter!
In the end ... do what works for you!