My wife is a photographer and takes issue with most LED's. Christmas lights, LED strips, etc. With LED strips, the problem occurs when you attempt to dim certain channels in order to attain a specific color. At full brightness, and close to full brightness, the LED flicker is not noticable at all, even to her. Which makes sense given the PWM.-- in simple case the PWM frequency won't need to be too high - just about the eye won't notice it or the interference of (incase the multi channel/-color)
If you compare stage-production LED lighting such as Chauvet Professional COLORdash Par-Quad 18, you can see in their specs that they use a 600 Hz PWM Frequency to achieve "flicker free" lighting.
Here's a relevant resource.