Ideally, yes. Otherwise, like with humans, the teeth can develop tartar, which can lead to bad breath. If things with their teeth get really bad, tooth loss might result.
You can use special toothpaste for cats and apply it with a small brush or even your finger (the brush would be better). Before doing that, though, it's a good idea to just let them lick the toothpaste off your finger for a few days, so they get used to the taste. Then use the brush/finger.
How old are your cats? If you get them used to brushing when they're very young, they're usually more accepting about it. I recently adopted 2 adult cats who have probably never had their teeth brushed (at least not by their owners) in their lives. I'm still trying to work up the courage to actually give it a shot.
There are also products out there that you just add to the cats' water. But somehow I don't think those work nearly as well as an actual brushing, just like mouthwash isn't a substitute for brushing for us.
We have a kitty that develops tartar and calculus like crazy. We brush her teeth, but we do not use toothpaste.
With her, we got lucky. She LOVED being brushed with a brush, and her favorite place to be brushed is on her cheeks. Because of the problem with her teeth, she loves having her gums massaged - so the transition to a small toothbrush that fits in her mouth was easy. She LOVES having her teeth brushed. The others don't - and we don't brush them. We do feed them treats once daily that help remove the tartar from their teeth.
...and we take the kitties for an annual dental. They remove the tartar build-up. Most vets don't recommend a dental annually until the kitties are older than like 5 or 6, I think.
i dont brush squishys teeth she wouldnt have anything to do with me putting a toothbrush in her mouth. but we do get her treats that are for dental hygiene.