I always request for the last row on Thunder Mountain Railroad, and have never been turned down. They'll say that I will have to wait and not be able to ride on the next one, but that's no big deal. Having the last row is worth the few minutes of wait.
I haven't requested a seat on Soarin' yet, mainly because I haven't had a bad seat so far, even when I'm in the 2nd row far left. I know of many people who request the first row, but y'know something, some time it's rather amusing to see people's feet dangling above you, especially when they left their sandals/flip-flops behind on the floor.
Zz.