I wouldn't be too worried about it if the tweeter is not at ear level, but it does depend on the design of the speaker to say how much it matters. First of all, the tweeter itself may or may not have narrow vertical dispersion. Dome tweeters and ring radiators have vertical dispersion that is equal to horizontal, of course, but ribbon tweeters, planar tweeters, and some AMTs will have fairly narrow vertical dispersion. The taller that the dimension of a tweeter is with respect to its width will be a restricting factor in the vertical dispersion.
Second, driver layout will affect vertical dispersion, especially with speakers with multiple midrange drivers. The further the midrange driver is from the tweeter, the more severe that the crossover null will be in the vertical off-axis. Also, MTM style arrangements create cancellation patterns off-axis, especially ones where the midrange drivers are not very close to the tweeter and/or have a relatively high crossover frequency.