When do you cover plants in MN?

In MN Spring is a blur between Winter and Summer. We had 80 degrees a week ago, the we had snow 2 days ago. I covered my perennials before the snow, now that the snow is gone can I leave them uncovered without the chance of having them freeze out? or leave them covered until it gets above freezing steadily? I am mainly worried about my Tulip and Daffodils that are 4 inches up already. Let me know your thoughts.