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Not much if they are dirty (and one must assume they are always dirty).

I beg to differ there.
Wooden pole ladders are used over here as a matter of course, by most Lines Companies on voltages of anywhere between 400V and 66kV.

Besides, if you're going to be climbing a pole ladder that will bring you within access distance of bare live lines (of any voltage), you'll be wearing gloves and outers.
And more than likely even fitting line hose over the conductors you aren't actually working on, but are in your immediate vicinity.

I'm not talking about special wooden ladders used by line companies but about ordinary wooden ladders.Couple of years ago man carried such ladder and ,as he passed below 110 kV power line, he was shocked when top of the ladder reached wires.

Things react differently when working at higher voltages.

At 480V, wood is an insulator; at 69kV, wood is a conductor.

I had this very discussion with a lineman around here about wooden hotsticks. I said that I thought they were non conductive, his answer, you just have to put enough voltage on it then it will conduct. Guess you learn something new every day.

I have stories about wooden ladders but the strongest punctuation on maintaining ladders comes from a friends recent experience. He was putting on siding on his house using a rather ancient extension ladder. Some ways up the lock broke and the extension slid down. His ankle got in between the two sections and some 3 months later he is walking and allowed to ride a wind trainer for exercise. His foot was all but torn off and it was only a couple of rather persistent tendons that saved his foot.

Another experience. The wooden extension ladder on my truck broke right where the wire reinforcing goes through the side rails, up some 4 to 6 inches from the bottom of the rail. I was told to cut the other rail off at the same height. I asked the boss if I should put feet on it as the wire rail is now right at the bottom.
Frugal as my boss was, he suggested that the wire is fine and we never need feet. Fine words, and not 3 months later his son in law had the same ladder on a polished concrete floor and the metal wire reinforcement acted like skates and allowed the ladder to slide away from the wall. The son in law missed a few days or weeks of work and I installed feet on 3 extension ladders the next day.
Lessons learned. The cheaper the ladder the better your maintenance program better be. Wood ladders weaken with weather and age and hardware rusts especially on the roof of a service van or hanging from the fence in the back yard. electrical resistance of a wooden ladder is also effected by the number of days it has rained.
On an unrelated to ladders note, I can tell you that wood, even a very dry hardwood broom handle conducts electricity very well