actually, it's to one's own peril - too dry and a torn frenulum is the result, helluva painful especially when urine contacts said area.
a little spit helps and directly from mouth to vag entrance is ok with doggie and she doesn't see it.
if frontal or missionary, it's a no-no - especially if it's a guttural-sounding haack-ptui to bring out the phlegm
just some no-nok etiquette to note