SSIS ETL Question/Option
We do this, the environment varaiable is at the user level, so the user for each environment that runs the SQL agent jobs is different. So on the the dev environment our agent user is something like SQLDev and on the QA environment it is something like SQLQA.

If you are not running from jobs (which I highly suggest doing except on dev while doing actual development), yes you have to change the enviroment variable to the correct one. We have created cmd line scripts to do that easily and placed them on our desktops, so that we can easily switch the environment variables.