If you've decided you would keep the baby either way, you may decide that you'll be more prepared knowing, or that it makes no difference whatsoever so you might as well wait until the birth to find out...
However, if you're avoiding the tests because you think that it would cause uneccessary worry during your pregnancy, it is worth considering whether you'd be able to cope with the shock
if anything was wrong with the baby once you had it... These test can be a great way of giving you a bit of time to adjust to the idea of having a baby with a disability, and can allow you to prepare yourself and decide how you and your OH will take to parenting under the circumstances. They can also provide a lot of reassurance if you are worried about the possibility of Downs syndrome or other disabilities.
However, like anita665 said, the tests can sometimes give you a high risk result when there's nothing wrong with your baby, or may come back as low risk, when there is a disability, which can be misleading and leads to unecessary worry, or in some cases to women terminating pregnancies that were perfectly healthy based on an incorrect probability.
Don't forget, that these tests work on probability - they can't give you a positive or negative result. Take them as a rough guideline (if you take them at all)... at most they can tell you to be prepared for a possibility - but we should all do that whether we have the tests or not