I haven't done much grout, but am starting to push it a little more lately.
The more I do, which is mainly residential bathrooms and kitchens, the more I realize how important it is to closely inspect the grout and pre-qualify the situation throughly with the customer and set their expectations as needed.
The customer has to understand that grout is POROUS and although completely removing surface soil is possible, anything that has penetrated below the surface may remain and present as discolorations, and may be out of reach of even acids.
Repeated use of consumer acid cleaners, especially right around the toilet bowl in bathrooms tend to leave that area appearing the cleanest, when in reality it is more due the fact that repeated exposure to acids has eaten the grout lower than the surrounding grout, and has exposed fresh grout.
Can't let the customer assume that this "cleaner" area if present around the bowl is an indicator of the potential level of improvement of the surrounding areas.
I find most customers ignorant to the characteristics and vulnerability of porous grout and often way out of line with their expectations, and need to be set straight on the facts before proceeding.
Gotta cover your arse with grout cleaning.
Hand and stick brushes and little wire brushes do come in awfully handy.