The screen widths Bootstrap v3.x uses are as follows:
Extra small devices
Phones (<768px)
/ .col-xs-
Small devices
Tablets (=768px)
/ .col-sm-
Medium devices
Desktops (=992px)
/ .col-md-
Large devices
Desktops (=1200px)
/ .col-lg-
So, these are good to use and work well in practice.