One volt does not matter. Two, sometimes.

One volt does not matter. Two, sometimes. As an engineer I accepted standards and never questioned them. A 5 volt circuit needs 5 volts. Don't run it on 4 volts or 6 volts. After years of working with teachers and students new to electronics I am finding that keeping to engineering specifications only serves to make STEM/STEAM projects less accessible. 

Using a 3v coin cell to power a 2 volt or 5v LED is just fine. Forget the resistor, the battery has enough internal resistance on it's own. The 2 volt red LED will shine mighty bright for a long time and the 5 volt LED will shine less bright for even longer (days). You can take this further with powering the robust Arduino and most old school integrated circuits. 

Jim Keith at Electro Schematics wrote an amazing article summarizing his work testing the venerable 555 IC. I have found similar results with over voltages. Here is the reality: when working with inexpensive electronic components and the goal of engagement and education it is better to worry less about one volt and focus on getting teachers and students picking up and using components to build projects that make them smile.