You can use a 2x4 at the bottom or at the top either way works. The only thing that you have to think about is shimming. If you have to shim a lot the 2x4 will get in the way. Another way you can do it is by installing the base cabinets first and getting them level. Then you and make a support that sits on the top of the base cabinet that will allow you to rest the upper cabinet on it to get the same height for each one. This way will allow you to shim with no problems.