Texas doesn't require testing, but I have started having the boys do the Stanford Achievement Tests every May. We've only done it two years.
I have them tested for the grade they would have been in if they were in public school (for you Brits that's the state run schools). They're both doing a year above that in homeschool. My husband suggested they take the test for the schoolwork they are doing. I don't know though.
I guess it depends on why I am having them tested. Originally it was to make sure we had done something right and to have some standardized proof that they were doing well. I got that. I guess now I should switch them to the grade they are doing. But I am afraid they might not do as well and I will feel bad. (Considering E got post high school on almost everything, I think I should be okay, though.)
I did find that M needs to work on library skills (that's the next unit in his English book) and spelling. But in almost everything else he is above average.
I think I might have them take two tests next year. No. That's twelve hours of testing just so I can be happy. I'll have to decide which one I want to do. (Hope I remember this whole thought sequence when sign-up time comes around.)