sucking a cock can be fun and wonderful thing--especially if know what to do--when sucking the dick without deepthroaitng it--breath through nose and sometimes through mouth-- take mouth off cock for couple seconds then breathe then slide it back into mouth --learning pacing and stuff will help you suck dick on your own pace--
if can control yourself--you can breathe while deepthroating a cock through mouth--sometimes guys have trouble as they have a gag reflex that really kicks in--and thats normal as the throat is made to try dislodging anything if gets choked--sometimes it can get whatever its choked by out sometimes it cant--so if control the gag reflex from kicking in will be fun for you and him
some guys like myself have full control --i dont have a gag reflex--so when a guy grabs my head and forces his cock down my throat holds it there can be quite exhileratingand then in 69 position me on my back him on top i love it when the fuck me mouth and throat slide their cock i n all way and shoot their

down my throat--feeling that dick throbbing in my throat as it does is awesome feeling as wel
so in my opinion its not the matter of breathing through nose or mouth--its up to you to get your rhythmn set to it--but with me i do both--suck on it breathe through nose then take mouth off a second breathe through mouth then go down on his dick as far as can --