when should i use braid and when shoul i use mono? i know i need braid on my frog rod for fishing thick weeds, but what about my 3 other baitcasters and my 4 spinning rods?

"Need" is way too strong of a word.  I don't use braid at all and manage to fish the salad at lakes all over Texas.  I have no trouble with XT.  It's also easier on my rods and reels.  

Just remember that there is a trade off for each trait.  

consider your water/cover/fish and then decide what is most important (rank the items in order)

strength

abraision resistance

visability

stretch

managability

line diameter

anything else you feel is important

Once you rank these things based on what baits you throw on each rod, you will probably not need our help on what makes most sense.

I have caught 5 bass over 10 lbs, a 11 lb 3 oz, & a 12 lb 8 oz this year alone; all were caught on 15 pound test Berkley Big Game and all were caught in thick vegetation.  

On the other side of the coin.  I run braid on all of my baitcasters.  When I am using a craw or jig I put a couple of feet of fluro leader on.  I like the feel that braid gives me.