I tend not to shift too much either, but a tight 90-degree, or even a decreasing-radius turn, if pretty severe, may necessitate shifting 2-1 if you can't corner with enough momentum. At Solo2 Nationals, the South Course had a severe right-hander that when I kept it at 2nd in my first run, I was losing so much time staying at 2nd well below my powerband. My second run, I shifted 2nd to 1st, and back to 2nd right after I rounded the sweeper. I ended up gaining about a second.