I think it depends on the context and the relationship with the client. I wait to be sure they'll perceive it in a positive light.
The majority of my clients do appreciate knowing if I pray for them, and it's a way to add another connection between you. In many businesses, the personal connection is just as important as the business purpose - you have opportunities to talk about kids, vacations, hobbies, etc.
In my line of work (taxes), it helps because part of the process is asking about donations to charities such as churches. That gives me a pretty good indication of what people believe and how they'll respond.

