I'd let Shaun chime in here, but most acid side cleaners designed to etch the top layer of grout, typically are quite a bit less potent than the say 6% or so of Hydrochloric acid that makes up most muriatic acids. The acid is going to keep working until its reacted with enough material to neutralize the formula. So more acid the better chance you could weaken the entire grout joint rather than etching off simply the top layer.
Most grout should be cleaned on the alkaline side for the soils and if its sealed from the get go and maintained correctly, an acid side cleaning should be reserved for a professional cleaning and only after an alkaline cleaning has been performed.
Also Acid is only for porcelain and ceramic with a sanded grout, can't remember off top of head if it reacts badly with the epoxy grouts on the market but I don't remember them being very favorable.
And Acid is a big no no for calcium based stones. Acid etches and Alkaline cleans the oils.. guess its depends on how badly the customer wants to redo their grout down the road.