I will usually do it. It's either that, or they take it to the car wash. With a lot of regular customers, I find they want a full detail every 4-7 weeks, and when I myself am washing the car, it makes the full detail a lot easier and quicker. I know that since I washed it, there will be no swirls to buff out, no really dirty interiors, and the wheels won't be covered in brake dust. That way, I'll be spending less time on the full detail while still making the same amount, giving me more time to do another full detail for another customer that day.
I have everything organized. For instance, I like to do washes in the areas south of me on Wenesdays. That way I don't have to do extra driving to make another $35, it's all local.
Plus, it definetly builds up loyalty with the customer. If you turn down washes, they might find another detailer that will be willing to, and before you know it, that detailer will be doing full details for them. I'd hate to admit it, but this is how I got a few customers.