Yes! By all means and ABSOLUTELY use sketch ups free models for reference. However, I really do suggest that you practice building your own models, as there will be a point when you're going to need something really specific and the free models won't work.
My comic uses Sketch up to create the backgrounds (though 95% of our sets are modeled, rather than free stuff out of necessity). My husband takes screen caps of the model at certain angles and matches them to the character art. He then uses Illustrator to vector and greyscale the backgrounds, and then it goes to color and texture in photoshop.
Here's a few examples of our sets:
We also use it for props and vehicles which is suuuuper helpful!