I got a rush ticket maybe an hour before curtain, and got a good rear orchestra seat. That was a Wednesday evening show back in June though, before the closing notice. Since Fiddler has been and is still on TDF (especially for weekday evenings), I suspect that plenty of rush seats should still be available and you may not have to show up right at 10 am...
I rushed Fiddler on Saturday 7/23 and got to the theater at around 9. There were probably 8 or so people ahead of me and after a little while more people came to line up behind me. My seats were in the Front Mezz row E and they were great!
Actually, I also rushed on 7/23. We got there at 9:10, and I think there were about 15 people ahead of us. We ended up with Front Mezz, Row A, seats 14-16 for the 8pm performance. The seats were fantastic. Super easy rush, at least currently.