do you have to put backing on all spinning reels even with those that are called braid ready spools?

I don't see why you wouldn't want to. Braids works sooooooooooo much better when it has very small diameter backing .. I use 4lb test for back on almost all my reels . 

Welcome aboard, Edwin!

No, but you'll need to either put a piece of
tape on the spool or something else to hold
the braid from slipping while you wind.

Welcome to BR Edwin!  I put mono backing on my spinning reels for two reasons:

1) I either don't have (or don't trust) "braid-ready" spools.  So I put mono backing on to keep the braid from slipping on the spool.

2) I fill the first half of the spool with mono so I use less braid.  I can normally fill 2 spinning reels with a 150yd. spool of braid.

You don't have to but usually I use backing to save on braid. Unless it's a very shallow spool.

Guess I was the only to reply to your other post with
the same topic, so in case that is deleted, here's the 
reply I posted there:

Welcome aboard, Edwin!

No, but you'll need to either put a piece of
tape on the spool or something else to hold
the braid from slipping while you wind.

 

Welcome!

even on "braid ready spools" i'll put a little bit on there, just makes me a little more confident about no slippage...

Just wrap it around the spool 5 or 6 times and then uni knot it, it'll never slip.  No need for backing, tape, nothing. The line will grip on to the wraps under the knot. Utube Spanish Fly, he has a video explaining it.

Braid is expensive and bass don't run far, so I use backing (kite string to take up space), and a uni-knot on top of the string.

I either use mono to back the braid to prevent slipping/not waste braid, or, I'll use 2-3 of those little strips they stick in the PowerPro boxes.  I'm assuming those are used to put on the spool to prevent slipping, or I've been using those things incorrectly lol.