Sheldon was pretty conservative about tire sizing, even by his own admission. I see 33 - 35mm tires pretty standard on all the CX racing bikes on the weekends. I just took off a pair of (labeled) 33mm that were (actual) 35mm tires. I frequently roll a pair of (labeled) 37mm that measure almost spot on.
The only issues that I've encountered with significantly larger tires (35mm+) is that you get a squishy feel in corners at speed, or on off-camber sections of terrain. On your rims I wouldn't worry about going with a tire up to 35mm (actual measure).

For the type of mixed terrain your commute has, check out the Schwalbe Marathon XR if you can find them around. I think they were discontinued, but they're a good, stout all-terrain touring tire. The Conti Travel Contact or Country Plus would also make for a good ride on either terrain, IMO.