In the US, because of the Magnuson-Moss Warranty Act, specifically the language regarding "tie-in sales", no automobile manufacturer can require that you bring the car back to them for the warranty to remain in effect.
(See
http://www.ftc.gov/bcp/conline/pubs/buspubs/warranty/undermag.htm
for more details)
They can, however, require that the service be done by a competent entity and that parts conform to their standards. Personally, I do all my own service, and I
a) keep detailed records of what, when, how, why
b) use only genuine OEM parts while under warranty
c) buy and use the best service manual available (in this case Helm's)
d) buy and use good quality tools
e) did I say I keep ALL records?
In about 15 years of doing my own car maintenance, warranty has never been an issue. YMMV.
Now regarding the extended warranty, remember that this is like an insurance policy, and the same rules apply. Insurance policies are written to make the insurance companies money, which means that for any single policy, the statistical "expected value" is in the insurance company's favor, i.e. if you purchase an extended warranty, the statistical expected value of the services you would get would be less than the cost of the warranty. Of course the big point is that this expected value is averaged over many thousands of policies, so if you did have to have repairs on your car, the cost of these repairs might greatly exceed the warranty cost. So as in every insurance decision, the issue boils down to one of emergency cash flow - i.e. if in the unlikely event of your needing large scale vehicle repairs (e.g. a new transmission at $3500), is it going to break your bank? If the answer is yes, buy the warranty. If the answer is no, you are better off not buying the warranty, i.e. being "self insured", and taking the chance that this cash flow outlay will never be required.