The first thing is that if you are using a smartphone, you are not using Excel (except maybe on a Windows phone). So the design approach for this must be as varied as the specific platform.

I'm a software development manager and we jump through hoops just to design web pages that will load properly on different known browsers. Trying to write an Excel file that will work on unknown platforms is even a bigger problem.

In short, avoid all the bells and whistles--especially macros. Some functions that experienced Excel users think of as essential don't even work on some mobile spreadsheet apps. Developing on Google Docs can be a good first step towards filtering out stuff that won't work, but you'll never know for sure without developing on the specific target platform.

This wouldn't have been as big a problem in 2003 but now Excel has become much more sophisticated and mobile knock-offs just haven't kept up.