With an SS, pedal strike is no more or less of a problem than with a geared bike, unlike with a FG where there is no option to coast around corners, so there is less reason to use shorter cranks. Personally, I still prefer shorter cranks on a SS for road riding. I normally use 172.5-175 mm cranks on a road bike, and 167.5-170 mm cranks on singlespeeds and fixies. Your cadence is far less constant on a one-geared bike compared to a multi geared bike and having shorter cranks increases the upper limit you can hold a comfortable cadence.
I have never heard of anyone using longer cranks for extra leverage on a road SS. Some SS MTBers use longer cranks, however, I've done a lot of SS and fixed riding on and off road and I've never felt like I needed longer cranks in comparison to a geared bike. I always feel like the same length or shorter is better, again because of the higher cadences. Generally, rather than increasing crank length, you gear the bike appropriately to climb the steepest hills.