The differences are so tiny it's not really worth worrying about. You should use ByVal whenever you don't need to modify the value.

Private m_NumAssignments As Long

Private Sub ByteByRef(ByRef X As Byte)
Dim i As Long

    For i = 1 To m_NumAssignments
        X = 123
    Next i
End Sub

Private Sub IntegerByRef(ByRef X As Integer)
Dim i As Long

    For i = 1 To m_NumAssignments
        X = 123
    Next i
End Sub

Private Sub ByteByVal(ByVal X As Byte)
Dim i As Long

    For i = 1 To m_NumAssignments
        X = 123
    Next i
End Sub

Private Sub IntegerByVal(ByVal X As Integer)
Dim i As Long

    For i = 1 To m_NumAssignments
        X = 123
    Next i
End Sub

Private Sub cmdGo_Click()
Dim num_trials As Long
Dim start_time As Single
Dim stop_time As Single
Dim i As Long
Dim value_integer As Integer
Dim value_byte As Byte

    lblIntegerByRef.Caption = ""
    lblIntegerByVal.Caption = ""
    lblByteByRef.Caption = ""
    lblByteByVal.Caption = ""
    Screen.MousePointer = vbHourglass
    DoEvents

    ' Integers.
    num_trials = CLng(txtIterations.Text)
    m_NumAssignments = CLng(txtAssignments.Text)
    start_time = Timer
    For i = 1 To num_trials
        IntegerByRef value_integer
    Next i
    stop_time = Timer
    lblIntegerByRef.Caption = Format$(stop_time - _
        start_time, "0.00")
    DoEvents

    start_time = Timer
    For i = 1 To num_trials
        IntegerByVal value_integer
    Next i
    stop_time = Timer
    lblIntegerByVal.Caption = Format$(stop_time - _
        start_time, "0.00")
    DoEvents

    ' Bytes.
    start_time = Timer
    For i = 1 To num_trials
        ByteByRef value_byte
    Next i
    stop_time = Timer
    lblByteByRef.Caption = Format$(stop_time - start_time, _
        "0.00")
    DoEvents

    start_time = Timer
    For i = 1 To num_trials
        ByteByVal value_byte
    Next i
    stop_time = Timer
    lblByteByVal.Caption = Format$(stop_time - start_time, _
        "0.00")
    DoEvents

    Screen.MousePointer = vbDefault
End Sub