Is it OK to leave a dimmer switch on low constantly (LED light)
There shouldn't be any dangers if you use a dimmer that's designed for LED lights. There are different types of dimmers for different types of lights. See this page from Lutron for a rough idea. Basically, most modern dimmers rapidly modulate between on/off and is safe for long term use in a low state. A side effect however, is that it may generate RF interference or even an audible humming sound.

In terms of danger, you might be thinking about very basic dimmers for hobby projects, which may use resistance dimming with a potentiometer. In that case, it's bad to keep it on in a dimmed state for a long period of time since it generates a lot of waste heat.

You should check this PDF (found on this page) by Leviton to see whether your specific LED light is compatible with your dimmer.