Vertical is the official Sonos recommendation, if I remember correctly.  Mine are vertically mounted.

In the vertical position each speaker is monaural and you’ll likely have better imaging if you sit and listen in a spot. In horizontal orientation, since each speaker has two tweeters, they’ll be used to create a wider sound stage as you move around in the room. Try both orientations. Only you will know what sounds “best” (to you).

Thanks for the feed back. Very useful