The answer to your question will depend on the specific code requirements of your municipality. Some only require that it (the subpanel) be grounded to a water pipe, and others may be fine if the sub and main panels are connected by metal conduit., but running a separate ground wire, in my opinion, is the best way to go. It ties the two panels together, ensuring that all the outlets are properly grounded. In my newly rewired home, both my main and sub are grounded to each other, and each to a water pipe. The main, in turn, is also grounded to an 8í grounding rod driven into the ground. Itís probably overkill, especially since on the panels you will notice that both the neutral and ground straps are connected, but thatís the way I like it. No matter what you do, you need to do the minimum (at least) required by your municipality, and make sure your work is inspected by a municipality-approved inspector, so that, if a problem comes up and your house should have an electrical fire or something, your insurance will pay.
As an FYI, I would also recommend evaluating your total power requirements for your house. A 60 amp sub is large compared to your main (my sub is 100, but my main is 200) If you truly believe youíll be pulling 60 amps just for the garage, then there wonít be much left for the rest of the house. If you have central air, that itself will pull 40-60 amps. If your house is all electric, then you also have an electric hot water heater (30 amps) and stove/oven (40-50 amps). Everything else in your house (fridge, microwave, fans, lights, etc) , in a typical day may take up an additional 20 amps. You could be blowing your mains regularly, which is a pain, especially if they are fuses and not breakers. Then, I would suggest upgrading the main panel to 200 amps.