If I leave a light on at night, does this help or hurt the grass? No particular kind of grass, just grass. I'm using CFL light bulbs.
Your porch light is going to be at least 4-5' away from the grass. It should have very little effect on it one way or the other.
As you probably know, light is very important to plants. Light helps them make their food which they need to grow and thrive. It's so important that plants will actually grow towards the light. This phenomenon is called phototropism.
With some plants phototropism is easy to spot. If you pay attention to a sunflower throughout the day, the flower part will always try and face the sun as it moves throughout the sky.
With grass it's not as easy to spot.
I have some direct experience observing the effects of small, artificial light sources on grass that reinforces my answer.
Last year I wanted to test some grass I was interested in and decided to record the grass growing.
You can see the phototropism of grass in the first 42 seconds of the time-lapse video I posted on my site. During time I didn't have a light on the plant. In the morning it would get light from a nearby window and it would grow towards the side where the window was. It was in my office so when I would come in and turn on the overhead lights it would then grow straight up towards the overhead light.
Eventually I placed a 25w incandescent bulb to take better pictures for the video. I left it on continuously. It was about 1' away from the nearest blades of grass. The grass seedlings were no longer waving back and forth with changes in light but overall they did have a distinct lean towards the light. There was more of a lean in the grass plants closest to the light than there was furthest away.
As you get further away from light the intensity is less. It is not linear meaning 2' away from a light is not 1/2 as much light as 1' away. It follows the inverse-square law so 2' away is actually 1/4 of the light that 1' away gets, at 4' it will be 1/16th. So there won't be much light getting to the grass unless you have a very powerful lightbulb.
Light is good for plants but your porch light isn't going to be providing a significant amount of light to make a noticeable difference. I also can't find any information that indicates plants require darkness. In parts of Alaska where the sun can be up for months at a time plants seem to grow just fine.