No, those ones just get dried and then lightly dampened before use.
Retting is really only used to break out fibres for spinning, like flax/nettle/rammie or to seperate the layers to make sheets of bast fibres from bark such as small leaved lime.
The grasses I used have no long phloem cell strands, the long fine fibres from flax/nettle that we spin.
Most folks who use nettle just peel off the skin which holds the fibres together and turn that into string, but retting breaks out the fine fibres that we spin into thread which we can weave into fine cloth.
Does that help any?
atb,
M