>For some reason, sometimes my reports get a specific printer set inside the FRX "DEVICE=..."
>How can I force my REPORT (.frx) to use the default printer?

You need to make sure to not save Printer Environment with your report. This mark should be unchecked when you open the report for modifications.

There was a bug that when you change font in the report, this check gets automatically checked. I don't recall if it was fixed in SP2 or not.