Ive seen some people have problems and others not. it really depends on the FA and their mood. If you can board early, you can probably put it in the overhead or ask the FA to store it in the FC cabin.
Just remember it has to be stored and can not be somewhere where it can move, i.e. in your lap, resting against a seat, etc. A lot of flights I take from SFO always seem to have one or two people with a guitar and people boarding in zone 3 or 4 usually have to check it because of limited space.
Good luck tho