Will there always be studs in the corner of two walls?
Those are all reasonable assumptions. But, you never know for certain whether anything was built by someone who knew what they were doing. Or whether it was inspected. 

I'd guess those assumptions are accurate 90% of the time?

Safe? Well, that depends on what you are doing in those areas. I wouldn't hang anything heavy that could fall and hurt someone if the assumption were not true (without 1st verifying the assumption).