It depends on what exactly you're doing. When tv or movies use fake brands it's more a matter of advertising, or really, not wanting to advertise. Think about it this way. If Homer Simpson drank Budweiser instead of Duff it would have some effect on Budweiser sales. While some shows are actually sponcered by certain brands. It could be subtle, like a character driving a Ford car, or completely in your face, like the almost omnipresent coca cola logo on American Idol.
Also the context of what you're doing matters. I make a video game/nerd culture comic, so at times I need to have products like the Xbox, or Wii in my comic, because I'm writing an actual commentary on them. However stuff like the brand of tv or computers used in the comic are never mentioned because really it has no importance to the plot. So feel free to use whatever products you want in order to tell your story, as long as you're not malicious about it you'll be fine.
In your case you're making a webcomic, so you probably don't have a major corporation sponcering you, nor is the spread of your comic so vast that if you had real world products in your comic it would influence actual purchasing trends, so do what you want to do. If you want to make up fake brands for the sake of quirkiness, do that. If you want to casually include real brands to give your comic some ties to the real world, do that, just don't go overboard to the point that it's intrusive, you might potentially alienate a part of the audience in doing so.