how often you de-thatch depends on the current level of present thatch. Having/leaving some is good for the lawn, (1/2"). anything more and I would de-thatch it.
I have only done a few thatch removal jobs myself. How ever I power rake to a tune of about $10,000 every spring.
I consider power raking to be a light comb of the lawn surface, only removing any dead stems and any debris that accumulated over the winter.
If your customer had a lot of thatch, this is what I would advise you try to educate them..... Stop bagging their clipping, leaving them on the lawn will help break down the thatch below them. Aerate 1-2 time a year to make sure air is able to get to the roots and the underside of any thatch present, again helping it to breakdown. If done correct think of it as adding compost (composted grass) to your lawn. This will ideal real estate for worms to live which naturally add compost and aerate the soil in a way we never could.
EDIT: I am in cool season grass which 5 months of snow so of course I have ZERO experience with warm season.