It's worth it is you need more RAM. I'm sorry that sounds a little obvious, but it's true. More RAM can speed things up, but if you're not doing anything with your computer that makes it run slow now, you don't need it.
If nothing else, buying more RAM from Apple is definitely NOT worth it. You can buy it separately from another manufacturer after you buy your computer from Apple and save a lot of money.
So, buy your computer, use it, and see if you need to buy more RAM.