Sorry, I'm back... yes, I "cheat" on fresh sourdough starters and use whatever yeast is handy (sometimes a pinch of commercial bread yeast, or bottle dregs if I have them).
For sourdough, you want a balance of yeast and bacteria. Bacteria is easy... flour is lousy with bacteria. Getting yeast out of the air can be tricky. Cheating by introducing yeast right away means you're going to have both in your starter.
Now truly great sourdough cultures are really complex, with lots going on. When you first start out with added yeast, the resulting starter isn't that complex. But if you feed your starter (essentially throw out most of it, refresh with fresh water and flour, and let it grow for a couple of days), it gets more complex. Enough variation will naturally happen that it will end up sourdough instead of just a yeasted blob (poolish is the actual bakers term). The trick is to not neglect it and let it dry out or get moldy (this is what happens to me once in a while).
You can also tweak the end result by timing when you use this starter... you'll notice that when you refresh it, it initially has a big yeast growth phase, where any bread made with it will be "cleaner" and yeastier. However, after the initial yeast period, the bacteria starts building, and it gets sourer, and funkier. So for a relatively tame sourdough, refresh your starter and then use it for your bread within a day. For wilder tasting sourdough, wait for two or three days after refreshing to use it.