If you install applications from testing and development?
The answer is yes they should, and most developers will try to make that aim. But to give a guarantee that it will always work?

The purpose of most devel repo's is to allow early testing, and it's pretty much accepted if you go down this road (on any platform) you are likely to hose your install and need to do a fresh one at some point.

testing is designed for UAT/QA testing and whilst packages there shouldn't hose your device, you still should be prepared for it.

packages in extras shouldn't need a re-flash, but as with all platforms and all systems, no testing system is 100% perfect - otherwise there would be no need for ticketing and bug systems.