board first and just put it in the overhead bin. it's an ... move because you're taking up 3-4 suitcase spots, i don't like it and probably 1/5 people also won't like it but also it's a musical instrument and we know united breaks guitars so whatever, that's the way she goes. but i guess a stickler could force a gate check and then the care of that guitar is entirely out of your control
if you're on a wide body you will be fine, tons of room in the cabin