If a fabric goes limp after washing - it probably wasn't all that wonderful anyway. (Unless soft is what one is wanting)
If one wants to use sizing or starch to stabilize a fabric for cutting - fine, I guess - but if one is using it to "beef up" a fabric - it will go floppy again the next time it's washed.
I wash a fabric to get it to be in it's more or less "natural" state -
then I have a very good idea of how it will be when it's "in use"