I commute with two bikes. One is my race bike, Standard Double, 10-speed cassette. The other is a single-speed mountain bike.
Its a no brainer for the single-speed.
For the roadie, I just choose whatever gear I am comfortable with in the conditions I am riding in. Flat is around 53- 17 or 19. If the grade goes up, I'll pop it up a gear or two. I try to maintain a steady cadance. On the major hills, I'm in the 39-19, 21, 23. I rarely use my 25. The 53-25 cross often derails so I avoid it at all costs, and the 39-25 is too low. I still wouldn't give it up because there are some hills (20-25%) that I need it on (training rides) and for when I am really tired.