TMS FlexCel v6.17 introduced full support for reading and saving strict xlsx files. Strict xlsx files are a different file format from normal xlsx files, and you can select them when doing a "File Save As..." in Excel:

Now the question is: Should I use normal or strict xlsx files? And the short answer is: Use normal xlsx files unless you have a non negotiable requirement from your customers to use strict xlsx.

The long answer can be found here.