

Duplicate the cutter and repeat for all the corners you wish.Select the original path and the cutter and do a boolean difference, if your corner is convex, a boolean union if it is concave.If necessary, rotate the cutter along its centre to align the arc with the edges of the underlying path.Activate snapping of centres of objects and cusp nodes to align the cutter exactly where you want. Duplicate your cutter to be able to reuse it and align it put it on the corner that you want to round.This ensures that the centre of your cutter will be aligned with the arc edges: Put the centre of the circle on one of the square corners and size the square as the circle diametre. Subtract a circle having the radius you want from a square to create a 'cutter' shape.

This works best with paths composed by orthogonal lines that are parallel to the axis. Once i got used to it, and learned how to use the snapping options, I find that I could work pretty fast.
