Here is a typical degradation curve. You have nothing to worry about!
How long do LEDs last?
LED luminaire useful life is often described by the number of
operating hours until the LED luminaire is emitting 70 percent of its
initial light output. Good-quality white LED lighting products are
expected to have a useful life of 30,000 to 50,000 hours or even
Source: US Dept of Energy
NOTE: This is applicable to LED light bulbs. The lighting surface of a light bulb LED gets HOT, too hot to touch. When an LED is run cool (under 50°C, it will last A LOT longer.
My LED light bulbs are guaranteed to last for 23 years. At 20 mA your LED is going to last a lot longer than 23 years.
Is it negligible or worth it?
Negligible. At 20 mA the heat is insignificant. It's heat that accelerates the mortality curve of any semiconductor, LEDs included. If kept under 50°C you are not going to have any heat issues.
Blue 0.02 amp x 3.2V = 64 mW
Green 0.02 amp x 3.2V = 64 mW
Red 0.02 amp x 2V = 40 mW
Total =168 mW
At least 20% of the wattage is radiant watts (light) so
168 mW - 20% = 134 mW heat., probably under 100 mW.
Any degradation will not be perceptible in your lifetime.
Select the intensity by measuring the temperature.
Run all colors at 20 mA and if you can hold your finger on the LED with no discomfort everything is fine.
If you burn your finger or you have to remove your finger after a few seconds then it is too hot and decrease the current.
IF too hot, Rinse and Repeat.
I have been designing LED lighting for years now. The LEDs you are using will not get hot.
Below is a 22" strip sitting on top my monitor. Vf = 46V x 0.250 amp = 11.5 watts / 48 LEDs = 240 mW/LED, about twice the max of your LED. The temperature of the LED is 41°C, slightly warm to the touch.
I guarantee you I can run this strip 24/7 for 5 years (43,000 hrs) and you will NOT perceive any difference in intensity.