1) Donot turn on swap.
2) If there's ever any problem with memory, create a swap file (if you don't have one yet) and type swapon on a live system.
1) is not the best idea, at least for Linux. One of the things the Linux kernel can effectively use swap for is defragmenting memory! Swap a chunk of memory out to disk, and read it back in at a more suitable location. A long-running, hard working kernel will reward you even for a measly 256M swap partition - eventually.
2) Swap files are significantly slower than swap devices. Consider this: Your swap file lives on a filesystem. Your system decides it's struggling with memory and begins trying to swap some cached files out (it may assume that swapping them out and back in again is faster than re-reading off the original disk)... potentially onto the same disk the originals come from, causing more thrashing. It may even have to consider whether it can to swap out bits of the swap file! Plus you're dealing with filesystem layers which will only slow down the process... so give it at least one device/partition as a priority, and add swap files with a lower priority for temporary/emergency purposes only, if you care about performance.
The amount of swap any system requires is very situation/application dependent. If your system isn't using swap much at all, then good for you! Don't throw a lot of swap at it! But be nice to your Linux kernel, and give it something to use for its own sanity.