Use unsigned indexes in MySQL?

No.

So simply "No" without any arguments?
As I recall, unsigned int improves the performance indexes, and mysqlperformanceblog.com there was an article about this (can't find now exactly, but here www.mysqlperformanceblog.com/2008/12/22/high-performance-click-analysis-with-mysql/ for example here is on looking for unsigned). You can still read their book High Performance Mysql, preferably 2 part.