It's instructive to hook up a timing light, connect a mighty-vac to the line to the distributor, and watch what happens. You don't get any advance until you have a few inches of vaccuum--I forget exactly where it kicks in, but this stuff is well documented on the web. You can have plenty of vaccuum at the carb port, but there still can be a problem with the advance mechanism in the distributor, which could prevent the advance from working. This is what happened to me. The distributor's vaccuum diaphragm thingy (pardon the high-level technical terminology) was frozen. Fortunately, it is available and not too expensive.
I didn't see any real difference in mileage or performance after I fixed it, but I don't do much highway driving, either.