There's no button for "yes, unless I don't". Generally, I have to look both ways before I even realize it's one way. Even for streets I know are one way, I'm pretty much looking all around - idiots can strike from any direction.
Yes, out of habit more than anything else. Also I'm teaching my son to "look both ways before you cross the street" which is much easier to parse for a 5 year-old than "look both ways unless it is a one-way, and if so then look for the one-way arrow and turn your head in the opposite direction." :)
I work on N. 1st St. in San Jose, which is a one-way street. Just the other day I walked out of my office, and lo and behold, there's some idiot driving the wrong way, who almost took out a guy on a motorcycle. It's not uncommon to also see people driving down the light rail tracks on that stretch of N. 1st. You better betcha I look both ways on that street!