Top down vs Bottom up
The key to a good responsive design is to remember the user wants a pleasant experience. Top down design gives you the opportunity to work with a lot of digital real estate. Being able to include all of the relevant information makes this process easier.
"Design is intelligence made visible" Alina Wheeler
The process of designing 'Bottom up' means you can consider the NBU's and people who move around a lot. This changes your entire perspective on what is important to your product. The only thing you have to remember for both approaches is that the user will always come first.