Performance-wise, swap space is not really an extension to normal, physical RAM. Whether your machines will need swap space or not is largely determined by what role they are fullfilling. For example, it is recommended for machines running an Oracle database to have some amount of swap space (Oracle has official recommendations for this), no matter the amount of RAM installed.
ESX can do over committing of memory, so if you allocate 6GiB of RAM to each machine, it does not mean each machine actually consumes each bit of that 6GiB.
ESX will also swap out memory on its own. ESX will create a swap file per VM and, if need be, will swap out memory of the VM to disk. To the VM, it will still seem like the memory blocks are located in RAM, even though they will come from disk.
If you can spare the RAM and your VM's require it, allocate 6GiB to them. This does not mean they will not benefit from having a swap partition inside the VM though. By default, Linux will try to swap out unused memory. This is defined by the "vm.swapiness" sysctl parameter.
To make a long story short, just listing the options will not give you a one size, fits all answer to how to configure your RAM. Tell us more: purpose, hardware, etc.