You may not leave a trace, but others may not be so cautious and careful about their footprint in environmentally sensitive areas. This is especially important in the late snowmelt season where hiking on the thin snowpack can damage the fragile meadows underneath. In addition you will run the risk of being given a ticket by a Park Ranger for violating the rules (which all visitors accept when entering the NP).
And you can get more information and locations from the photo guide wildflowers Web page. Enjoy them and take care to let others enjoy them too.