Vertical dispersion of a speaker is usually much poorer than the horizontal dispersion. Where a good speaker design may allow great leniency in audibility across the soundstage... once you stand up... that's the bugaboo!
In general, I'd say that within 15º off axis is a necessity for most speakers in vertical dispersion. This gets complicated by XO design even more as sometimes you will find speakers with an upward tilt to the radiating pattern.
It is always well worth experimenting with placement.
My advice is to know how high your ear-holes are when you sit at your LP... how high is it to the center of the tweeter... How far you sit... Do a little MATH: High-School Geometry! You can figure that out pretty easily, if you are within the sweet spot.