What is the relation between the question on the subject and if there is a need to test all widgetsets?
Well, I use Mandriva, and I generally don't test a software with all widgetsets possible. I just need one that works for each platform. For one software I may use win32 for windows, gtk2 for linux and qt for Mac OS X.
For another project I may decide to go, for example, with win32 on windows, gtk1 on linux and carbon on Mac OS X
1 widgetset for each platform you support is more then enougth.