Using Bootstrap 3 and saw that there is a revamped grid system (col-xs, col-sm, col-md, col-lg). I can see that sometimes you will want to have a few columns when viewing a site on a mobile phone or portrait mode tablet and hence the reason why someone may want to use col-xs-2 and col-sm-2 or whatever. However, I like to keep things simple: is it best to just stick with the "col-md-x" class and design the site with that in mind? So, any device viewport smaller than 970px will stack vertically (col-md breakpoint is 969px) . Personally, I think MOST designs look ugly and cluttered when you have more than one column on pages less than 750px.
Is the "col-md-x" class is a good baseline for a responsive site? For design purposes, it seems the most manageable and easiest to work with.