I'm thinking about using mono to back the braid on my baitcaster

You just need a few wraps, not much.

I have backed some reels, but others I haven't. I have never had a problem with slip.

I back braid with mono. I also re spool a real backwards to get the good braid on top.

I usually spool about halfway with mono. Then top with 75yards of braid. I almost always fish with a fluorocarbon leader so I never lose braid. I've had the same braid on reels for 5 years or more with no issues. As someone else said, when the end of it gets worn, spool that braid backwards onto another reel and you're good to go for another long while.

Thanks. What kind of knot do you use to back your mono on the braid?

Uni

3 feet or so is what my sons do...

I back mine with mono. Tie a uni to uni knot and use about 75-90 yards braid. Super slick 8 power pro is $20 a spool. I can do 2 reels that price. Nothing I fish in LA (specks, reds, bass) will take that much line. Plus the uni to uni knot is super strong and would hold in that situation anyway.

I back with mono, but then put a wrap or two of electrical tape on the mono. Then I tie the braid like I would an empty spool. I do the same thing with mono or braid especially on my bigger reels. When I re-spool before a tournament I'm usually doing 15 or so reels and it saves a lot of money on line.

I do this on most of mine. Uni to uni knot.

I used to then stopped when I realized there was no point outside of saving some braid, and I'm only concerned with that on my flipping lines where I don't need much.