Trev,
I only usually offer this to the larger houses I do; £20 plus type houses.
Since I work with Wor Lass, we have what may seem strange working practises. She normally just works till 2 or 3 pm, and in places where I have compact work.
We normally work where I WFP tops only and she does the ground floor trad, so I like to have a few £20 plus houses I can do after dropping her off home; normally 100% WFP, rather than tops only.