That's not supported out of the box. You could align nodes in response to NodeCreated and NodeModified handlers, with the help of the AlignPointToGrid method. Once a node is created or moved to a new location, find the grid point nearest to the node center, and offset the node so that its center coincides with the grid point.

I hope that helps,
Stoyan

Thanks, I think I might be able to make that work.

Though, just in case anyone else ever needs this in order to calculate allignedCenter in Stoyo's code call AlignPointToGrid not AlignToGrid.

The code I'm using currently looks like:
diagram.AlignToGrid = true;
Point ptCenter = node.GetCenter();
Point ptAlignedCenter = diagram.AlignPointToGrid(ptCenter);
double fDeltaX = ptAlignedCenter.X - ptCenter.X;
double fDeltaY = ptAlignedCenter.Y - ptCenter.Y;
diagram.AlignToGrid = false;

Rect rectBounds = node.Bounds;
rectBounds.Offset(fDeltaX, fDeltaY);
node.Bounds = rectBounds;