Rent a car unless you are absolutely sure you are doing NOTHING but the convention. Pasadena is actually a very nice town for restaurants and cute stores. If you are not in the mood to drive around, just take the taxi, hire car, or flyaway bus/gold line to and from the airport as others have mentioned. You will find plenty of places to eat and spend your money in Pasadena all in a centralized district.

But if you want to see anything in LA, or venture out of Pasadena after the convention, you will need car. No question about it. Even with a car, I might make Pasadena my home base and go touring from there. There is really no reason to stay in downtown LA or Hollywood, you will have to fight the traffic to the convention, and then you'll still not be where you want to be. Believe me, there is no "center" of LA that's convenient to everything.

The streets and freeways are very well marked, and traffic is not any worse than any major city. A GPS is a great investment for a nervous driver.