I'm not using a validation rule, I set the MinValue and MaxValue properties on my NumericTextBox. If I enter a value outside that range, then the cell shows the red border, and the red icon appears over the row header. I realize now that there is a a tooltip message, but it only appears when I hover over the cell. That marker on the row header doesn't show the tooltip. Maybe that is good, if a row has multiple errors, which tooltip would it show? I don't know, I haven't worked with the grid for very long.
In any event, the tooltip over the cell is not very helpful. It says "The value "" could not be converted". Of course what I would like it to say is something like "The value entered is less that the MinValue property" or something. I just went ahead and did a cell validation rule, so I can supply my own custom message (which would be better, anyway, than the one in the previous sentence). I removed the MinValue and MaxValue settings on the NumericTextBox and delegate all validation to this rule.