you could i guess
especially if you want a contrasting color in there from the stem up partway
but i think it's more efficient to do it at the end
after it's dried & assembled i usually just slide the brush down in there between the petals a bit
you don't want it too dusty either to interfere with any gluing to be done either
the wet (from glue) dust might make a noticeable color difference
there are others much more qualified than me but that's how i do it
