You could use a formula something like:
Dim x as Number
x = 1.56
If Remainder (x, 0.05) < 0.025 Then
Formula = x - Remainder (x, 0.05)
Else
Formula = x + (0.05 - Remainder (x, 0.05))
End If
If you can have negative number than you will have to modify this slightly to work.