Ideally speakers are vertical. Their sound dispersion is better that way. Originally THX specs required a vertical cent for that reason, but vertical centers are simply impractical. If it was me, I'd find a way to have them vertical, but it isn't the end of the world if you don't.
There are some instances where that's the case, but it's the exception rather than the rule. I see no reason that would be the case with these speakers. Dome tweeters (which they have) radiate evenly in a full circle.