Jockey wheels - does it matter if they're worn?

[Thias]

The upper is the guide.

 

[xerxes]

Re:

Worn ones definitely mess up the shifting. I had a mech with really worn jockey wheels and I couldn't get the shifting to index across the whole cassette, only 3 or 4 sprockets out of the seven, replaced the jockey wheels, tweaked the tension screw and it shifted perfectly.