If this is your first time sampling and you have reason to believe the history of one part of the lawn is different from the rest, then I'd do two samples. Get the test done by Logan Labs for $25 per test. After that you can determine whether one sample is enough for next year. Most likely the soil will test close enough to the same, but if you do two tests then you'll know for sure.

If money is a factor then you can just mix it all up and send in one sample.

If you need more convincing to go with a Logan Labs test rather than your local county or state university, just ask. Or look for soil test results here on this forum to see the quality of replies you get from a Logan Labs test.