Software Deployment

not only ini file. ini, inf, config, conf, xml, log, cfg ect these extention files may modified in application run time. 

If any key path files are miss or modified msi will get self-help and put exact key file from MSI. So skip this you would remove key for these files.

Depends.  If the file is the only thing in a component, then Yes, use it since a component should have a key path.  If it's not the only thing in a component, then probably not - but not all the time  :)

If the config file is modified by the application regularly, you may not want to set it as a key path since the key path is what will trigger self-heal.  Odds are pretty good that the config file will not have version numbering so it wouldn't be used, but it might be triggered by the timestamp if for some reason the timestamp of your config file is set in the future.  A long-shot?  You bet.  But look into the logic used by the engine for triggering self-heal to understand what's going to happen in your scenario.