I was curious about how often not prefetching tasks helps to get a 1st, so I looked at the range including my most recent 100 validated workunits for which I got 1st (without prefetching) for GFN-16. They take me about 5:11 each.
216 total WUs
118 (54.6%) 1st (or still pending), and would have been 1st even if I had prefetched (that is, I returned task at least 2:30 before wingman)
89 (41.2%) 2nd
9 (4.2%) 1st, but would have been 2nd had I prefetched (that is, I returned task less than 2:30 before wingman)
Conclusion: for roughly 95% of workunits, the server decides who gets 1st.
I'm not certain how this scales to longer tasks.
Edit: I looked at BOINC manager as a task was completing, and it seemed to take roughly 8 seconds between completing one task and starting the next. This is about 2.6% of the time it takes me to complete one task, so if I had prefetched, I would have been able to do an extra 5 WUs in the time it took to do the 216 above. Maybe better to turn it back on...less selfish too...