Join our CAD community forums where over 25,000 users interact to solve day to day problems and share ideas. We encourage you to visit, invite you to participate and look forward to your input and opinions. Acrobat 3D, AutoCAD, Catia, Inventor, IronCAD, Creo, Pro/ENGINEER, Solid Edge, SolidWorks, and others.
The default value 2 mm or 0.125 in for fillet/chamfer shows up only for the first time you invoke the command during an entire Inventor session. For all subsequent dialogs, the fillet/chamfer displays the MRUV (most recently used value). I don't think you can change the default values.