All of the so-called "rules" about swap size were never intended to be rules. They were simplifications, little guidelines that could help you guesstimate a decent number if you didn't know the RAM usage patterns of the box or just didn't care to optimize.
As time passed the simplifications became folklore and "rules" to be obeyed, and the relative cost & latency differences between RAM and disk gradually changed. Today most of these rules are at best meaningless.
However, note that the question isn't "I can keep all my apps in RAM, I don't need swap". The question is "I have x RAM. How do I best utilize it, for the needs of running apps, sleeping apps, and disk I/O buffering?".
OPs question is tagged "Linux", and I don't know that much about the Linux Virtual Memory Management (I know more about Windows and FreeBSD), so I will not offer up suggested new guidelines here. A good place to start reading would be this discussion thread between the Linux 2.6 kernel developers about the "swappiness" VM tuning parameter.
when you install Linux without swap, the installer will warn you that you didn't have swap mount.
That's just hardcoded behaviour in the installer, because having swap is almost always a good idea. Many Linux distros may need the swap for Hibernation, but you can change that if you want to.
No I just don't need it [swap] because my RAM is just big enough, right?
Wrong. Or correct for small amounts of "need", you strictly speaking don't need swap in this case, but your PC will be faster with it. Without swap, all your programs reside in RAM, including the ones that are inactive and are not going to execute any time soon. With swap, inactive programs can be swapped out, and the physical RAM be used for more productive things -- mainly disk buffering to masquerade how slow disk I/O (incl SSD) really is.
If you want to control the kernels balancing between de-allocating disk buffers and swapping out application code, then you want the "swappiness" tuning parameter, a discussion of which I linked to above.