This just came up and I've been asked numerous times "when do I draw the line?" when it comes to a set of tests.  Here's a great example of where that line happens...

    [TestClass]
    public class ExportStory_Tests
    {
        private string fakeXml = @"
<rss version=""0.92"">
  <channel>     
    <item>
          <title>[TEST-1] Test</title>
          <type id=""1"" iconUrl=""https://someinstance/"">Story</type>
    </item>
    <item>
          <title>[TEST-2] Test</title>
          <type id=""2"" iconUrl=""https://someinstance/"">Backlog</type>
    </item>
    <item>
          <title>[TEST-3] Test</title>
          <type id=""3"" iconUrl=""https://someinstance/"">Defect</type>
    </item>
    <item>
          <title>[TEST-4] Test</title>
          <type id=""4"" iconUrl=""https://someinstance/"">Task</type>
    </item>
  </channel>
</rss>
";

        private ExportStories _exportStories;
        private IEnumerable<XElement> _result;

        [TestInitialize]
        public void Setup()
        {
            _exportStories = new ExportStories(null, null);
            var elements = XDocument.Parse(fakeStoryXml).XPathSelectElements("rss/channel/item");
            _result = _exportStories.GrabStories(elements, new[] { "story", "backlog" });
        }

        [TestMethod]
        public void grabs_two_stories()
        {
            _result.Count().ShouldEqual(2);
        }

        [TestMethod]
        public void grabs_test_one_and_test_two()
        {
            _result.First().Element("title").Value.ShouldEqual("[TEST-1] Test");
            _result.Last().Element("title").Value.ShouldEqual("[TEST-2] Test");
        }
    }

Somewhat simple, nothing fancy going on here -- all it tests is grabbing the right items via a filter (kinda?).  So I could've drawn the line at the first test.  It was 4 elements, now its two, so we're good to go !

Not so fast. I have two of them, but are they the right ones?  What if my logic was 100% reversed so instead of two stories it grabbed the other 2 things?  Ok, so I could add in 1 more item to my test data and that would certainly hint that was correct ...but I preferred to take the inspection to the items directly so the second test checks for the titles.

Where I drew the line is here : "does the order matter, ever?" No, absolutely not.  I'm controlling whats going in and I know it's sequential.  I could write a bit of code to grab the two titles and ask if it contains what I expect -- but that's the line I decided to draw.