If you are asking me.
I power wash under the Jeep in October. Then I go to town under there. I mean everywhere.
Thick and heavy.
During the winter I run my Jeep through the $6 automatic bay at the car wash once in awhile.
I do not pay the extra $1 for the underside. Fluid Film will eventually wash off.
Come April power wash it all off.
I don't run it April to October because when working on a Fluid Filmed Jeep it is just messy as hell.
But if I ever get to a point where all I do is maintence (oil changes, etc) under there then I will probably
run Fluid Film year round.
Now after I power wash in April there are a few places I will hit with Fluid Film.
Pretty much anywhere there is a bushing.
Give the leaf springs a dose.
Also where the adjustment for the paking brake is.
Let's face it. That is bad the way it is in the open like that.

 

Not sure where you are located, but I do a dumber/lazier version of what Ralph77 described. If they salt the roads in your area, then you might think about hitting the common rust areas with fluid film a couple of times each winter. I do more of a spot protection/lube. I hit:
*Inside lip of front fenders (there is a fold on the inside of front of fender that holds all kinds of madness that rust loves)
*along rocker (including brake lines, evap hoses, fuel line, etc) Fluid Film doesn't melt plastic or rubber
*pinch weld on bottom of doors
*trans & t-case linkage
*between leafs
*parking brake mech and drum springs/hardware
*rear wiper spring and hinge
*rear latch assembly and inside lock

Edit: wanted to add that I'm in Chicago and have fully in-tact rockers on my 2000.

 

WD 40 is the Army's 40'th try at a water displacement "product", no better a lubricant than pee. (I haven't personally tested to see if that stops squeaks!). Arisol chain and cable lube, (lithium?) is ****. Shoots so far you can lube your hinges from across the street!

BTW....for better of for worse, the bottle gear oil, 90W comes in with the nozzle makes using that easy, but the spray stuff, it sticks than as the bubbles break it soaks in well.

 

can't disagree with any of that ^... in my experience, the ladies prefer Fluid Film for its manly musk and earthy tones, over the lingering fishy flavors of 75-90 gear oil. something to keep in mind on those special date nights in the XJ... lol!

the gear oil is a good idea!

 

That's Funny! I did mean that I like spray lube, it wouldn't let me say ****, *****, *****, two mammary glands....And: Gotta break them in to the fragrance of 90W before they get to changing the fluids silly!

Weird....on my phone a minute ago "****" was ****. Now on a computer it's as I wrote. Wonder WTF is up with that.