If your mailmerge is being run through code, you could use that code to call the cleanup routine. There are more efficient means of doing that, though.
The following code will delete all blank rows (except for those with vertically merged cells) in all tables in the active document.
Code:
Sub DelBlankRows()
Application.ScreenUpdating = False
Dim i As Long, j As Long
With ActiveDocument
For i = .Tables.Count To 1 Step -1
With .Tables(i)
For j = .Rows.Count To 1 Step -1
On Error Resume Next 'skip vertically merged cells
With .Rows(j)
If Trim(Replace(Replace(.Range.Text, Chr(13) & Chr(7), ""), Chr(160), "")) = "" Then .Delete
End With
On Error GoTo 0
Next
End With
Next
End With
Application.ScreenUpdating = True
End Sub