Y'know, this has bothered me for a long time. I mean, I don't go through life upset about it, or anything, however, anytime I can, I try to promote "good rat image." I carry pictures of my boys, and talk about them and show off their pix a lot. Some people are very surprised to see that ~THAT~ is actually what a pet rat looks like.
Still, it's very ingrained in people to look down on rats, and it's just not fair. Anytime I hear one of those derogatory slang comments involving a rat, I bring it up, and prove how silly it would seem, with any other small, furry animal.
"He's nothing but a low-down, no-good, dirty chinchilla!"
"Yeah, when da cops got 'im, he hamstered us out."
"...like bunnies deserting a sinking ship."
"I smell a GERBIL!"
...and so on. Nobody takes as much collective verbal abuse as the rat, even if it's not directed at them personally, and considered a "figure of speech."