Boiler Tune-up

 Every year my heating service company conducts a tune up and there are 10 items listed that should be performed every year.

In the past 5 years they completed every item except, "clean gas burners and pilot".  This was only done once in 2007.

I asked the technician last month, if the burners are in good condition, after I noticed that he looked at the burners but did not clean them. He said yes.

I have a one pipe steam system. My boiler is a Weil Mclean, about 25-30 years old.  My question is how often do the burners and the pilot need to be cleaned?

Thanks