Having the snap toggle would allow the user to select whole groups of times easily, without typing that in manually or fiddling too much with the mouse. Selecting say, a 2x3 block would be as simple as one mouse gesture.
This would be a toggle setting in the EDIT menu , like "Snap to Mosiac Tile Borders" and greyed out if the project doesn't have Mosaic Tiles. When the setting was on, it would affect the Selection Mouse Mode only. When Snap was ON, selection boundaries created by the mouse would snap to nearby Mosiac borders. What was considered "nearby" would be a setting (in pixels or metres?) that could be changes somewhere in settings.