When I taught elementary school, I often pretested every math unit. During some years, it was a requirement, and as a grade level, we regrouped the students according to the pretests. Other years, it was simply something I did on my own, to guide my instruction.
When I started teaching math at the middle level, I sometimes pretested, but often found that most of my students needed instruction in the majority of the skills on the pretest. Because math classes are shorter in middle school than elementary, and because I always felt behind in the curriculum, I gradually stopped my pretesting practices.
Some students did not get to the more complicated problem solving before our unit test (it was on Friday), but that was ok...they had still completed enough practice of the concepts to be successful on the test.
Overall, I was pleased with the results...although some days were a little chaotic, especially with my largest class...which also happens to be the most needy class.
In the next couple of days, I plan to pretest the classes on the next unit (number theory and fractions); I'm very curious to see whether there is as much disparity in their current skill levels as there was in the decimal unit.