I do the same - I never call without knowing exactly what flights I've wanted. But agent roulette is a huge factor. US generally is very good with regard to what they see tends to match ANA Tool consistently, no Starnet worries. The good agents get the intinerary booked quickly and then it's just a fare desk issue. Even that's fast if it's a relatively simple itinerary.

However, another aspect of a good agent can actually slow the process down - this is a good thing. On a recent award, I could only find availability in Y on a late connecting flight. It was the domestic leg so I was going to take it. The agent thought my choice was odd and asked if I wanted the earlier connection. Took him an extra 25 minutes but he got me the early flight and 2 seats in F. In such a situation, if they can open award seats that ANA doesn't show then I'll wait all day.