The metal components of your drivetrain - chain, chainrings, cassette - are put under tremendous stresses when you pedal. They wear out over time because of this force, and they wear out in a pattern (but you already know this, because you're replacing all of those parts together).
Your plastic derailleur pulleys do not bear any of this force at all. They are there just to keep the chain moving along and don't wear much at all, comparatively speaking. If you clean your derailleur up really well, you may notice virtually no wear.
All that being said, they're cheap and easy to replace. If they look worn, replace them. If not, don't sweat it.
Roll them by hand and look at the tooth profile. Unless they feel very stiff or seized when rotating they are fine. The tooth profile should look similar to a cassete tooth . When the top flat part disappears and the teeth look sharp and pointy like shark teeth then its time to replace.
FWIW...I ran my XTR for many years with a pretty badly worn lower guide wheel and it still shifted pretty well. I noticed that the upper guide, closest to the cassette, still looked pretty good, so I swapped them....and it still works great!
Yeah, I know, that's the ultimate in cheap, like saving alum foil, but hey, it works!
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