It is really important, for many cars this is the intended return path for the spark current. A bad ground in some cars can result in heater core leaks.

I have seen (for instance) as the current finds an unintended path and melts the solder on the core inlet/outlet. The owner of the car did not know this until after the second heater core. In this case the connection was just dirty after 100,000 miles of use.

Another example… when the block ground strap broke. The current found the path of least resistance, which was through the emergency break cables and melted the plastic sleeves redering the E brake useless.

Bottom line, always make sure the block has a good ground.