Do I need a timer on my coffee grinder?
I have never used a timer myself and I never recommend using one.
The reason for this is that it is generally more accurate to dose by weight than to dose by grind time. This is because when grinding with a timer typical grind weight accuracy for an Italian coffee grinder is +/- 3 grams. When I dose by weight I aim for an accuracy of +/- 0.5 grams.
To learn more about dosing by weight see my Espresso Workflow