I grew up on "cross chain and die" but Shimano says it isn't really an issue with todays flexible 10speedchains. Indeed, I cross big-big occasionally (or even a hair more) and it doesn't sound or feel bad like it did in 1977 (or 1987)
Small-small on my bike doesn't work at all. The angle the 34 needs means the chain actually hits the 50 ring on small-small.
But riding in 50-25 or even 27 doen't look like that bad an angle, and it sounds fine, so heck, I'm doing it. I've gotta replace my new-improved planned wear-out 10-speed Ultegra chain every other day anyway...