When I taught beginners, I started everyone on the side. Most beginners are not tall enough to play properly with the horn in the center. When I taught HS, didn't really make lot's of difference with altos, in my opinion. But if I saw someone playing in the center with their hand resting on their leg, I would make them play to the side until they could play with correct posture and hand position. All things being equal, good hand position, posture, etc. I can't really be convinced that it makes any difference for Altos. I play Tenor in the center occassionaly.