Irena, I haven't read all of the previous responses, and we're in a different state, so my advice may be completely useless to you, but fwiw, I think I'd wait to think this through until next year, simply because you'll have some more time to see how the test prep etc unfolds. I realize that you'll need lead time to opt out, so definitely go through that process of figuring out what will be required, but I'd leave the actual decision for awhile. This is why - just based on our situation in our state - which of course, is probably entirely irrelevant for you!
* Our state testing is really not terribly stressful compared to other types of tests my kids have taken (TerraNova etc). It also is not very difficult at all - my dysgraphic ds who had extreme challenges with written expression *still* scored one point shy of advanced (without any accommodations) on his first round of state testing in 3rd grade. (Note: hopefully your state's tests are a little bit more challenging and meaningful - seriously!)
* Our school district does use the state testing for some types of academic placement - it's not the only criteria but it's part of it.
* Our school district adds in nationally normed standardized tests as the students progress in grade levels (TerraNova, ITBS) - and those tests are required for placement into gifted programs in secondary school. They are tests that are longer and more potentially stressful (or have been for my kids)... and they are the tests that count the most. I think the state testing was helpful for my kids to just get used to the general idea of standardized tests (in a group setting) before they had to take the TN etc. I also think it was helpful for my ds starting in 4th grade to get used to taking tests with his accommodations (he had them starting in 4th, didn't have them in 3rd). He was very self-conscious about feeling different at that point in time, and it was helpful for his first experiences with having a "quiet" room (room with less students) and word processor.
Best wishes,
polarbear
