I ground to anything that's connected to the car's unibody (that's metal of course!). I usually connect to the radiator support...wiggle the booster cable's clip to make sure I'm gripping metal and not dirt and grease. Don't connect to the engine block (common mistake) since the ground strap might be faulty and the engine is usually isolated from the subframe by the engine mounts. I've boosted lots of people's cars in the winter months here in Ottawa. Safety comes first, as always. I make sure that when I'm cranking the dead car over that everyone's behind the hood, shielded against exploding batteries. Not a winter goes by around here without a tale of some hapless soul that hasn't properly hooked up a battery through negligence or because they're too proud to ask, and a battery explodes.
BBQs in the summer. Batteries in the winter.
/Eric
