It depends on the content and, if you’re using a web design app to layout the pages, it can depend on the web app. Hype breakpoints handle a minimum width and expand up to the next widest breakpoint whereas some web design apps handle the breakpoint as a maximum width and shrink. That sort of mismatch can be a mess, particularly if your Hype document switches heights between breakpoints. This can cause elements to overlap on a lower resolution device.
Another thing to be aware of is that certain norms in the Windows world have carried forward from the days when the OS was very primitive (e.g., the 6-foot mouse cable standard was originally so it could connect to a mouse card on the back of a PC tower on the floor). A relevant example is that most Windows users keep the default window setting of full-screen (from the days when you could only run one program at a time), which means that the average Windows desktop user will tend view the content across a wider area than the average desktop Mac user.
Smartphone screen resolutions vary widely, with the smallest (the Sudroid J5) having a resolution of 240 x 320. The lowest popular resolution has a width of 480. Note that this refers to physical pixels, whereas CSS (virtual) resolution is as low as 320. Check out the following:
In my situation (which includes in-house design of corporate and retail sites - with the need to maximize conversions by maximizing the experience) I cover my bases and use four breakpoints, with uniform layout heights. Something to keep in mind that with narrow layouts, floating content can spread more than you might want as you reach the next largest breakpoint. I typically set the breakpoints at 320, 480, 768 and 1100, which work the way up on a ratio of 150% to 165%.