So, about to hit level 20

You should always care about runes; they can easily make the difference between winning/losing a lane. Spending IP on lower-tier runes before you hit 20 will set you back on IP.

Well, on a lark, I did get some, but only to try out the rune fusing thing. So ATM, I only have, like, 2.

Have you actually saved up any IP for runes?

If you're having fun and enjoying the game I wouldn't worry about it overmuch. Most people won't have full rune sets until they actually hit lvl30 and even if they do, they don't have "perfect" sets for all their champs. That said, it is a significant competitive advantage to have them.

If you already have a couple of your favorite champs, go ahead and start buying some runes for them. Start with the generic ones good for all champs like Armor Seals or Movement Speed Quints then go from there.

ATM, I have 4880 IP. I was considering using that to get another champ, but I have enough that I know how to use well, and more that I just don't know at all, so I decided to sit on it for now.

As for favorite champs? It depends on the lane I'm in, and in order of skill/enjoyment: sup>mid>top>adc>jung. I tend to main Nami for sup, but do well with morgana and soraka sup also. Mordekaiser I run top lane, usually (read: always now) with a tanky build, but I also have wukong (but have neglected to learn him). Mid I have Brand, and technically Yasuo, though I'm still learning him too. ADC I have Kayle (yes, it works more often than not, but it's tough some matches) and... I think ashe? not sure. Jung I have fiddlesticks and udyr that i'd default to.
Would that matter as far as runes actually go?