It's a good question. When I first learned to fish, I took it as a kind of rule that you plumbed up then set your rig to put the bait laid on, just on or fractionally off the bottom; that was the right presentation, I was told. It took me quite a long time to twig that that was limiting my options and that fish don't always want a bait in that way. So now I generally plumb carefully, mark the depth against rod or pole, and start on or near the bottom. But I don't think twice about sliding the float up to whatever depth to see what happens. A good clue to fish feeding at mid-depth is when you get bites on the drop. With the depth marked, you can always revert to full depth in a few seconds. Don't be afraid to experiment - I also used to think you put the bait in and left it; it's clear that lifting your bait out and letting it fall again is often the much better way.