I have flown 14 sectors on U2 so far this year split evenly between where I've paid for SB and where I haven't.
I pretty much always check-in immediately, or wait to the 30 day mark if I've booked that far out.
My non SB seat allocations have been 1 x A, 3 x D and 3 x F all between rows 16 and 24.
Ironically the only middle seat I've been allocated is for a flight next week where I've paid for SB, but the original flight was cancelled and I moved to an earlier (busier) one.