Modern systems have enough ram that swap is not much used. If it becomes necessary, the overhead of encrypting will be the least of your concerns. If your system is swapping, it's crawling anyway.

In theory anything could be in ram could also end up in swap. So yes, it is important. And yes, there is overhead, but it isn't too bad if your system isn't RAM constrained, and your CPU supports AES-NI or similar.

Ideally, you wouldn't even have a dedicated swap partition. No data is always more secure than even the most perfectly-encrypted data.

If you do have swap, though, then the encryption probably won't matter much compared to the fact that your machine is swapping in the first place.

Hibernation is a very nice feature (I use it for laptop and desktop machines) and it requires a swap partition. Also, a encrypted swap is even more important in this scenario.

In the laptop I'm typing this, swap is one of the partions over a LVM2 physical volume over LUKS.

That's why I sad "ideally" :)