Inevitably when pressure washing you will always remove some grout or sand. If it comes out it will need doing soon anyway.
Few tips though.
Always weed kill a few days before doing the job.
The more pressure you use the more of the surface layer of the slab you remove. This results in the slab soiling up a lot quicker.
Either use a steam cleaner, this means cleaning at high heat and low pressure thus removeing very little of the surface layer of the slab and cleaning very effectivly. Or after pressure washing let the surface dry, sweep and then roll on some patio cleaner availible from B&Q
James