Personally i do everything i can to avoid inside corners/fillet welds if they can't be left 'as is'... sheet metal and i turn the corner and use a butt weld instead for example
I bought that power file this week and very impressed so far, though only used it for paint and rust removal, not cleaning up welds. £41 at the the B and D shop at the Lowry, Salford. Dremmels are fantastic tools too. I use mine for all sorts of things, though not dressing welds so far.