What will happen with my applications and data if I upgrade to El Capitan GM?
I'd always go for safety over a bit of time-saving, every single time.

Though 99 times out of 100 you'll have no issues upgrading the OS, there's always that one time something unexpected happens.

There's also the chance you don't like it, don't think it does something you've been used to being a certain way, or isn't compatible with an important app you need.

In any of those cases, not having a backup could spell disaster, or at least major discomfort.

Personally, I always have a current Time Machine backup before updating, &amp; I remove that Time Machine drive &amp; replace it with another just before I actually hit the update button; I also clone the boot drive first, so I can quickly revert.  

You might think that is overkill, but I've never lost any data yet.
Last time I did it was just last week - when I updated to the El Capitan GM &amp; didn't like it at all ;-)