I got a mh 7'0' ft Shakespeare agility from "santa" and I'm gonna use it for mat fishing, I want to use braid but I don't know if I need monofilament backing or not. can you guys help?

YES.

Backing isn't needed if you use tape, I haven't used backing in years.

Braid will spin on your spool sometimes without a backing. Spinning sucks and if it happens you will be throwing away that whole spool of braid. So what do you think? Backing or no backing?

Backing isn't needed if you use tape, I haven't used backing in years.

Tape? Do you put the tape around the spool and then line it or do you put it on the line after it's already been attached to the spool?

I used to use a piece of tape like they sell in a pharmacy for taping up wounds, it's sort of like a cloth tape, one strip around the spool with very minimal overlap of the ends was enough to stop all slipping. 

I use painters tape (masking). Depending on what I'm fishing for I do it 1 of 2 ways. Larger fish and saltwater I lay the line on the bare spool and put the tape over it, I would rather lose a lure and a spool of line than have some bruiser spool me then do damage to the reel by knotting it down, I'm an eye witness to this.

If I'm lazy I do the same thing in freshwater, but usually apply tape to bare spool then knot it down over the tape. I only use spinning and only use braid, never had a slippage issue.