No, SRO is only sold when the house is sold out although depending on the person selling the tickets, they're sometimes sold when there are seats left. But if you get the guy, he's super mean and will refuse to sell if there's even only one seat left 5 minutes before curtain.
Honestly, that makes no sense to me. Because one seat worth around $100 isn't sold, he denies the house of anywhere between an extra $25 and $300. But whatever, it's the house's loss.
Waiting for tickets to Hair August 2008