Yes, I'm sorry if I haven't been clear in some of my responses. It's a good idea to set these out clearly.
- How should the grid work when you drag pieces around?
Push the piece to the nearest multiple of 8 (or whatever the grid size).
- How should the grid work when moving pieces with the keys?
Arrow keys (with no modifier) should move the piece by 8, rather than to a multiple of 8. This allows rapid tiling with a single piece (duplicate, move by 8, duplicate, move by 8 etc) even when the tiling does not start on a multiple of 8 (sometimes necessary to fit in with irregular terrain).
I love Dullstar's recent suggestion that, when the grid is on, ctrl-arrow should move the piece by 1. This allows slight movement without the hassle of turning the grid off for a single move and then turning it back on.
- How should the grid work when duplicating a piece?
Either the current method (duplicate at same location) or Lix's method (duplicate at (x+8,y+8)) are fine. I have no preference between these, so maybe stick with the current method as it's what NL users are used to? (The Lix method has a slight disadvantage that sometimes you lose pieces off the bottom of the level.)
- How should the grid work when inserting a piece previously copied to the clipboard?
At the moment it pastes at the same location as the copied piece, and I don't see any reason to change that.