With the cursor in an input field you can ctrl+z to undo, or right click in the field for a pop-up menu which has an undo option. The undo only toggles back and forth between undo and redo though - it's not multiple undos (which I quite like, as mostly you're comparing values). This will work for as many parameters as needed, but once you bring up the settings pane for a new node (not included floating panes) then the undo memory is wiped. Sorry if this was already obvious...
Aside: - it'd be great if as part of the menu options for each node's parameter, there was an option to save and recall the current value as a preset (perhaps even 2 or 3 presets?) which remained local to that node instance. I know that you can already save out nodes as clips, but that's more of a permanent action. I'm thinking here that being able to quickly save and recall presets would speed things up when experimenting with settings - no need to keep them around permanently once you've found something that you like.